Add leader-key (multi-key sequence) shortcuts — foundation (#431)

[cloud-on-prem]

Summary

Adds configurable leader-key + multi-key sequence shortcuts (e.g. ⌘K then t n) as an additive, opt-in layer alongside the existing single-chord shortcutOverrides. Closes the foundation for #431.

Sequences are lowered to Ghostty's native sequence-keybind engine (leader>k>k=action) and injected at launch through the existing GhosttyCLI.argv → ghostty_init path — no new input layer.

Scope of this PR (foundation, v1)

This is deliberately a Swift-only foundation PR. While investigating, I confirmed a load-bearing constraint:

A leader sequence lowered to Ghostty's engine can only trigger Ghostty built-in actions that round-trip to the host (rt_app.performAction). text:/esc:/csi: keybind actions route to the pty (child process), not the host, and apprt.Action.Key has a fixed member set with no generic host passthrough.

So the issue's menu-only app actions (Create/Archive Worktree, Toggle Sidebar, Open PR, Open Repo, Reveal in Finder, Next/Prev Worktree) cannot be bound to a sequence without an out-of-tree Ghostty source patch. Rather than ship that patch in the first PR, this PR builds the entire mechanism and binds the actions that work natively today, with a clean, additive seam for the rest.

In this PR

• Configurable leader chord (default ⌘K, verified free in-app), persisted in a new additive GlobalSettings.leaderKey.
• Multi-key sequences → Ghostty leader>k>k=action keybind lowering.
• Settings → Shortcuts UI: leader row + sequence editor + target picker.
• Prefix/duplicate/leader/reserved/single-chord conflict warnings (inline, non-blocking).
• Backward-compatible, zero-migration persistence — existing single chords untouched.
• A bound leader>escape=end_key_sequence cancel; unmatched sequences pass through to the terminal natively.

Bindable actions in v1 (Ghostty host-routable built-ins): New/Close Tab, Jump-to-Worktree-N (goto_tab:N), Move Tab, Toggle Command Palette, and the five split actions. Menu-only app actions are structurally unrepresentable in the target picker so they can't silently no-op.

Deferred (follow-ups)

• Menu-only app-action targets — via an out-of-tree Ghostty patch adding a generic host-routable action; the stored LeaderActionTarget is an extensible sum type (.ghostty(...) today, .appShortcut(...) later) and the codec lossy-drops unknown kinds, so the follow-up is zero-migration.
• which-key discoverability overlay (a documented extension seam only).
• No app-side sequence timeout — Ghostty has none (no config, no mid-sequence host callback), so unmatched sequences rely on native flush + the explicit escape cancel.

Implementation

Area	File
Model types	SupacodeSettingsShared/App/LeaderKeySequence.swift (new)
Persistence	SupacodeSettingsShared/Models/GlobalSettings.swift
Keybind lowering + launch wiring	SupacodeSettingsShared/App/AppShortcuts.swift, supacode/App/supacodeApp.swift
Conflict validator (pure, trie-based)	SupacodeSettingsShared/App/LeaderKeyConflictValidator.swift (new)
Reducer arms	SupacodeSettingsFeature/Reducer/SettingsFeature.swift
Settings UI	supacode/Features/Settings/Views/KeyboardShortcutsSettingsView.swift, SequenceKeyRecorder.swift (new)
Tests (37 cases)	supacodeTests/ (model codec/lossy, lowering strings, validator, reducer TestStore, backward-compat)

Testing

• 37 unit tests covering: Codable round-trips + lossy-drop, backward-compat decode (leaderKey == nil for pre-feature settings), lowering strings (incl. the leader>escape=end_key_sequence cancel), all conflict cases, and the reducer arms via TestStore.
• SwiftLint --strict: 0 violations across all changed files.

⚠️ Not yet build-verified — remaining gate

I could not build/run the app or execute the test bundle in my environment: Frameworks/GhosttyKit.xcframework is absent and a from-source zig build fails here with libSystem link errors (environment/toolchain, unrelated to this change). The code is verified by swiftc -typecheck (against real types where possible) + SwiftLint + the authored test suite, but the full-graph compile and runtime behavior have not been exercised. Please run, before merge:

• make build-app and make test (executes the 5 new test classes) in a build-capable environment / CI
• Bind a sequence to a built-in (e.g. goto_tab, a split) and confirm it fires while Supacode is focused
• Multi-step sequence waits for the 2nd key without leaking the first to the terminal
• After the leader, an unmatched key flushes to the shell and the prompt stays usable (and doesn't strand a TUI like less)
• leader then escape cancels cleanly
• A single chord and a sequence on the same action both fire independently
• Leader binds are injected at launch, so changes apply after relaunch (expected; surfaced in the UI)

🤖 Generated using: 🎮 rp1.run