refactor(#3308): add planning-snapshot.cts parsed projection per ADR-3180 §8.1

Phase 10 of epic #3180. src/planning-snapshot.cts is a new parsed
projection of .planning/, composed exclusively from the already-
consolidated §7 owners (getMilestoneInfo, listMilestonePhaseDirs,
isPhaseComplete, scanPhasePlans, stateFieldValue, planningPaths) plus
the frozen SCOPE enum. No new semantic derivation is introduced beyond
worstScope, a pure combinator folding several independently-scoped
owner answers into one composite signal.

Adds STATE_UNREADABLE to src/unusable-input.cts's UNUSABLE_REASON
(seventh #1879 site) for STATE.md exists-but-unreadable, distinct
from absent.

Adds scripts/lint-planning-snapshot-bypass-drift.cjs, a ratcheted
drift guard (ADR-3180 Decision 4(e)) scoped to DIAGNOSTIC_RULE_FUNCTIONS
(currently cmdValidateHealth in src/verify.cts only) preventing new
raw .planning/ reads from bypassing the snapshot, while acknowledging
cmdValidateHealth's existing 15 raw-read sites as debt owned by
Phase 11 (#3309).

Six-gate .cts ripple: .gitignore, eslint.config.mjs,
docs/INVENTORY.md + manifest regen, CONTEXT.md glossary entry.

Breaking changes: none. This phase adds the subject only; Phase 11
migrates cmdValidateHealth onto it.

package.json

@@ -114,7 +114,7 @@
     "lint:table-schema-drift": "node scripts/lint-table-schema-drift.cjs",
     "lint:frontmatter-scalar-broad-grep": "node scripts/lint-frontmatter-scalar-broad-grep.cjs",
     "lint:removed-but-needed": "node scripts/lint-removed-but-needed.cjs",
-    "lint:ci": "npm run lint && npm run lint:skill-deps && npm run lint:generated-sync && node scripts/lint-test-file-count.cjs && node scripts/lint-command-contract.cjs && node scripts/lint-pr-check-project-dir.cjs && npm run lint:legacy-name && node scripts/lint-regression-test-names.cjs && node scripts/lint-allow-test-rule-refs.cjs && node scripts/lint-resolution-provenance.cjs && node scripts/lint-emitted-drift-ack.cjs && node scripts/lint-portable-timeout.cjs && node scripts/validate-registry.cjs && node scripts/lint-table-schema-drift.cjs && node scripts/lint-fix-has-regression-test.cjs && node scripts/lint-example-parser-parity.cjs && node scripts/lint-docs-command-form.cjs && node scripts/lint-plan-count-drift.cjs && node scripts/lint-milestone-window-drift.cjs && node scripts/lint-phase-enumeration-drift.cjs && node scripts/lint-planning-prompt-drift.cjs && node scripts/lint-completion-ratio-drift.cjs && node scripts/lint-state-field-drift.cjs && node scripts/lint-completion-predicate-drift.cjs && node scripts/lint-frontmatter-scalar-broad-grep.cjs && node scripts/lint-removed-but-needed.cjs",
+    "lint:ci": "npm run lint && npm run lint:skill-deps && npm run lint:generated-sync && node scripts/lint-test-file-count.cjs && node scripts/lint-command-contract.cjs && node scripts/lint-pr-check-project-dir.cjs && npm run lint:legacy-name && node scripts/lint-regression-test-names.cjs && node scripts/lint-allow-test-rule-refs.cjs && node scripts/lint-resolution-provenance.cjs && node scripts/lint-emitted-drift-ack.cjs && node scripts/lint-portable-timeout.cjs && node scripts/validate-registry.cjs && node scripts/lint-table-schema-drift.cjs && node scripts/lint-fix-has-regression-test.cjs && node scripts/lint-example-parser-parity.cjs && node scripts/lint-docs-command-form.cjs && node scripts/lint-plan-count-drift.cjs && node scripts/lint-milestone-window-drift.cjs && node scripts/lint-phase-enumeration-drift.cjs && node scripts/lint-planning-prompt-drift.cjs && node scripts/lint-completion-ratio-drift.cjs && node scripts/lint-state-field-drift.cjs && node scripts/lint-completion-predicate-drift.cjs && node scripts/lint-planning-snapshot-bypass-drift.cjs && node scripts/lint-frontmatter-scalar-broad-grep.cjs && node scripts/lint-removed-but-needed.cjs",
     "lint:allow-test-rule-refs": "node scripts/lint-allow-test-rule-refs.cjs",
     "lint:regression-names": "node scripts/lint-regression-test-names.cjs",
     "lint:descriptions": "node scripts/lint-descriptions.cjs",