feat: use FSEvents for macOS file-watch triggers

[d0ugal]

Summary

• Add a daemon-internal file-watch backend interface and select FSEvents automatically on darwin && cgo, with fsnotify preserved everywhere else.
• Use one recursive FSEvents stream per watch binding while keeping matcher, ignore, debounce, retry, lifecycle, and trigger delivery semantics shared.
• Conservatively scan FSEvents coalesced/lossy subtrees, directory renames, moved-in trees, and missing-path subtree events, and document FSEvents latency, coalescing, and path-based rename limits.

Resource evidence

• On macOS/cgo, TestFSEventsBackendUsesSingleRecursiveStream creates a nested worktree and a broad **/*.go watch with watch_max_directories = 1; the binding stays healthy with watch_backend=fsevents, one recorded watch path, and daemon watch cost 1.
• The fsnotify backend remains directory-registration based, so non-FSEvents backends still charge their existing per-directory or macOS kqueue descriptor estimate.

Scope notes

• Backend selection is automatic in this v1; no user-facing backend switch was added.
• Existing watcher attribution from feat: show per-session watcher usage in trigger status #1749/feat: attribute trigger watcher usage #1752 now reports backend cost; this PR does not add a new diagnostics surface.

Fixes #1750

Test plan

• go test -v ./internal/daemon -run 'Test(FSEventsBackend|HandleWatchEvent_(CoalescedSubtreeScan|LossySubtreeScanNotesEligibleDirectory|LossyMissingSubtreeNotesEligibleDirectory|RenamedDirectoryScansSubtree|PreciseDirectoryScanDoesNotNoteEmptyEligibleSubtree)|FileWatchBudgetDegradesAndReleases|WatcherDiagnosticAttribution)$' -count=1
• go test -v ./internal/daemon -run 'TestFSEventsBackend(UsesSingleRecursiveStream|EndToEndNestedCreate|DirectoryRenameRequestsSubtreeScan)$' -count=1
• go test ./internal/cli -run 'TestCheckWatcherResources|TestTrigger' -count=1
• go test ./internal/config -count=1
• go test ./internal/protocol -count=1
• go build ./cmd/graith
• make lint-only
• git diff --check
• CI passes on latest push

Note: before the rebase onto #1752, the same focused daemon suite also passed under -race. A full local go test ./internal/daemon -count=1 attempt was interrupted after several minutes in unrelated lifecycle coverage without surfacing a concrete failure; CI remains the authoritative full-suite run.

[github-actions]

📸 Docs preview

Preview screenshots for this PR were cleaned up after it was closed.

[github-actions]

📊 Test coverage

Component	Coverage	Δ vs base
Go (./...)	80.5%	+0.0%
Swift (gui/shared)	not run (no gui/ changes)	—

Per-file Go coverage (most uncovered first) — click to expand

File	Coverage	Statements	Δ vs base
internal/libghosttydeps/generate.go	11.4%	55/482	+0.0%
internal/cipolicy/artifact.go	74.2%	741/999	+0.0%
internal/cipolicy/p11_js_surface.go	73.0%	316/433	+0.0%
internal/config/config.go	92.6%	1098/1186	+0.0%
internal/cipolicy/plan.go	82.9%	372/449	+0.0%
internal/cipolicy/validate.go	79.2%	262/331	+0.0%
internal/libghosttyarchive/archive.go	73.7%	185/251	+0.0%
internal/cibaseline/inventory.go	84.7%	355/419	+0.0%
cmd/docspreview/main.go	39.2%	29/74	+0.0%
internal/agent/transcript/codex.go	85.7%	240/280	+0.0%
internal/docspreview/docspreview.go	86.7%	255/294	+0.0%
internal/agent/transcript/claude.go	81.2%	155/191	+0.0%
internal/cipolicy/io.go	56.2%	41/73	+0.0%
internal/sandbox/nono.go	82.7%	153/185	+0.0%
cmd/docsdiff/main.go	91.0%	314/345	+0.0%
internal/store/store.go	84.0%	163/194	+0.0%
internal/cipolicy/build.go	89.8%	229/255	+0.0%
internal/libghosttydeps/cmd/main.go	0.0%	0/26	+0.0%
cmd/architecturecheck/main.go	0.0%	0/25	+0.0%
internal/architecture/check.go	88.6%	171/193	+0.0%
internal/config/trigger.go	93.4%	310/332	+0.0%
internal/sandbox/safehouse.go	74.7%	65/87	+0.0%
internal/testutil/graith.go	63.3%	38/60	+0.0%
cmd/cipolicy/main.go	75.9%	66/87	+0.0%
internal/version/update.go	71.6%	53/74	+0.0%
internal/git/worktree.go	82.4%	89/108	+0.0%
internal/sandbox/denials.go	88.8%	151/170	+0.0%
internal/config/paths.go	71.4%	45/63	+0.0%
internal/capabilities/capabilities.go	91.9%	192/209	+0.0%
internal/cipolicy/credential.go	66.0%	33/50	+0.0%
internal/docspreview/github.go	87.4%	118/135	+0.0%
internal/protocol/manifest.go	78.5%	62/79	+0.0%
internal/executablepin/pin_linux.go	62.5%	25/40	+0.0%
internal/scenariofile/scenariofile.go	94.1%	240/255	+0.0%
internal/libghosttydeps/lock.go	86.3%	88/102	+0.0%
cmd/cibaseline/main.go	71.7%	33/46	+0.0%
internal/sandbox/why.go	80.9%	55/68	+0.0%
internal/cipolicy/manifest.go	90.2%	101/112	+0.0%
internal/libghosttydeps/verify.go	84.7%	61/72	+0.0%
internal/agent/transcript/render.go	87.7%	64/73	+0.0%
internal/atomicfile/atomicfile.go	81.6%	40/49	+0.0%
internal/ignore/ignore.go	82.7%	43/52	+0.0%
internal/config/watcher.go	78.9%	30/38	+0.0%
internal/sessionlabel/label.go	85.2%	46/54	+0.0%
cmd/libghosttyarchive/main.go	74.1%	20/27	+0.0%
internal/git/branch.go	82.1%	32/39	+0.0%
internal/agent/transcript/usage.go	88.5%	46/52	+0.0%
internal/protocol/messages.go	78.6%	22/28	+0.0%
internal/config/scenario.go	72.2%	13/18	+0.0%
internal/detector/detector.go	95.2%	100/105	+0.0%
internal/agent/transcript/scanner.go	91.5%	43/47	+0.0%
internal/config/render.go	92.7%	51/55	+0.0%
internal/sandbox/kernel_linux.go	0.0%	0/4	+0.0%
internal/sandbox/sandbox.go	84.0%	21/25	+0.0%
internal/git/maintenance.go	83.3%	15/18	+0.0%
internal/testutil/git.go	93.5%	43/46	+0.0%
internal/agent/transcript/transcript.go	92.3%	24/26	+0.0%
internal/git/git.go	97.5%	79/81	+0.0%
internal/output/output.go	89.5%	17/19	+0.0%
internal/scenariofile/protocol_config.go	97.7%	84/86	+0.0%
internal/tools/tools.go	96.0%	48/50	+0.0%
cmd/covreport/main.go	97.1%	34/35	+0.0%
internal/covreport/covreport.go	98.7%	77/78	+0.0%
internal/git/username.go	96.0%	24/25	+0.0%
internal/hookoutput/hookoutput.go	90.0%	9/10	+0.0%
internal/libghosttydeps/render.go	94.1%	16/17	+0.0%
internal/processidentity/processidentity.go	93.3%	14/15	+0.0%
internal/protocol/frame.go	95.5%	21/22	+0.0%
internal/testprocess/testprocess.go	90.9%	10/11	+0.0%
internal/agent/agent.go	100.0%	40/40	+0.0%
internal/agent/transcript/limits.go	100.0%	10/10	+0.0%
internal/config/template.go	100.0%	23/23	+0.0%
internal/config/trigger_template.go	100.0%	16/16	+0.0%
internal/cronx/cronx.go	100.0%	24/24	+0.0%
internal/scenariofile/policy.go	100.0%	64/64	+0.0%

_{Base coverage: Go 80.5%. Per-file figures are statement coverage. Swift shows an absolute % (no delta) to keep macOS CI light. Updated on every push.}