Add recall/promotion tracking fields to Entry schema

[quad341]

What changes

Adds 5 new fields to the Entry struct and matching SQLite columns to the
index so entries can track recall/promotion lifecycle state:

• Kind — distinguishes a plain note (default, empty) from a remediation entry
• AutoActionable — reviewer-granted flag for remediation entries that can be
  acted on automatically; not self-declarable by the entry's own creation path
• RecurrenceCount — incremented when a new entry is captured with the same
  topic key as an existing one
• PromotedBeadID — set once an entry is promoted to a tracked bead, guarding
  against double-promotion
• LastRecalledAt (RFC3339) — stamped only by the get/freshness/verify path,
  not by capture

All 5 fields are additive and zero-value-safe: existing entries with none of
this metadata parse and round-trip unchanged (omitempty/omitzero tags
throughout).

Why built this way

The index table gets the same 5 columns via a forward migration
(addColumnIfMissing), so existing on-disk indexes upgrade in place. The
three index-only fields (RecurrenceCount, PromotedBeadID, LastRecalledAt)
plus Kind/AutoActionable are deliberately excluded from reindex's
ON CONFLICT(id) DO UPDATE SET clause — the same treatment the existing
hit_count column already gets. Reindexing rebuilds the index from each
entry's body file, but these fields are written directly via SQL by other
call sites after an entry already exists; if reindex overwrote them on every
run, it would stamp live index-only state back to the body's stale seed
value. Excluding them from the UPDATE SET means insert-if-absent still works
for new rows, while existing rows keep whatever the index already has.

What to review

• internal/cairn/entry.go — the 5 new struct fields and their toml tags
• internal/cairn/index.go — new columns in the CREATE TABLE, the
  migration column list, and the reindexTx INSERT/upsert logic (note the
  bool→int conversion for AutoActionable, and the intentional gap in the
  DO UPDATE SET clause)

Test plan

• TestParseEntryNewFieldsZeroValues / TestParseEntryNewFieldsRoundTrip —
  parsing with none vs. all of the new fields present
• TestEntryMarshalRoundTripsNewFields / TestEntryMarshalOmitsZeroValueNewFields
  — serialization round-trips and confirms zero values stay omitted
• TestReindexPreservesNewIndexOnlyFields — reindex does not clobber
  index-only state written outside the body file
• TestReindexPopulatesNewFieldColumns — fresh rows get correct values
• TestReindexMigratesLegacyIndexSchemaNewFields — an old on-disk index
  without these columns migrates forward cleanly

All existing and new tests pass, including -race. gofmt, go vet, and
golangci-lint are clean.