fix(state): persist model assignments across sync runs

[christorres-hu]

Closes #264

Summary

• Fixes model assignments being silently reset to "balanced" preset on every sync run
• CLI sync and TUI sync now load persisted assignments from state.json when no flags/overrides are provided
• Read-merge-write pattern ensures InstalledAgents and other state fields are preserved
• Added 7 integration tests covering CLI load path, TUI round-trip, backward compat, and multi-sync cycle

Design note

claudeAliasesToStrings and modelAssignmentsToState are intentionally duplicated as unexported helpers in app and cli — avoids coupling state to model for two 8-line functions.

Test plan

• TestRunSyncLoadsPersistedModelAssignments — CLI sync loads from state.json
• TestRunSyncDoesNotOverridePersistedAssignmentsOnSecondSync — assignments survive sync1 → sync2
• TestRunSyncWithNoPersistedAssignmentsDoesNotPanic — backward compat with old state
• TestLoadPersistedAssignmentsPopulatesEmptySelection — TUI load path
• TestLoadPersistedAssignmentsDoesNotOverrideExisting — TUI overrides not clobbered
• TestPersistAssignmentsPreservesInstalledAgents — read-merge-write safety
• TestPersistAssignmentsNoOpWhenEmpty — no spurious writes
• Full suite: go test ./... — 37 packages, 0 failures

[Alan-TheGentleman]

Review: fix(state): persist model assignments across sync runs

Verdict: APPROVE

Clean, well-tested fix. The read-merge-write pattern is correct, backward compatibility is covered by the omitempty tags and the legacy-state test, and the 7 new tests cover the important scenarios (load, no-clobber, merge, no-op, multi-sync cycle, backward compat).

A few non-blocking observations:

1. Duplicated helpers — claudeAliasesToStrings / modelAssignmentsToState appear identically in both app and cli. Acknowledged in the PR description as intentional to avoid coupling. If a third caller appears, consider extracting to a shared internal package.

2. Inline vs. extracted load logic — The TUI path extracts loadPersistedAssignments as a clean helper, but sync.go has the same logic inline. Minor consistency nit.

3. TUI persists after sync, CLI does not — tuiSync calls persistAssignments after execution; RunSync does not. Currently harmless since nothing mutates assignments between load and execute, but the asymmetry is worth a comment for future maintainers.

4. ModTime-based no-op test — TestPersistAssignmentsNoOpWhenEmpty compares file timestamps. On coarse-grained filesystems this could theoretically be fragile. Content comparison would be more robust, but unlikely to flake in practice.

Overall: solid work, good test coverage, clear PR description. Ship it.