feat(compaction): use model-managed CLM by default

[MelodyVAR]

Long sessions currently go straight to native summary compaction when they cross the context threshold. This makes clm-v1 the default: the model first makes a bounded edit of eligible old observations in a working view, while the canonical session history stays intact.

Behavior

• Keep the existing native trigger (contextWindow - reserveTokens). CLM maintenance runs before a prompt or after a complete tool turn; a settled answer defers routine maintenance until another request needs the context.
• Validate edits atomically against their session, branch, source digest, and working revision. Preserve user messages, control metadata, reasoning, and tool linkage; archive and persist accepted revisions before using them. Resume, native compaction, and task-state snapshots retain the working context and remaining work.
• Reuse a matching actor request prefix, system prompt, and tool schemas for JSON maintenance. Cold or changed contexts use the isolated edit-tool transport. Maintenance never dispatches project tools. Physical cache hits still depend on provider behavior; edited prefixes invalidate subsequent cached tokens.
• Bound maintenance to two requests, 90 seconds, and 8,192 output tokens by default, with a three-turn cooldown and a minimum saving of both 1,024 tokens and 5%. No useful edit, rejection, or failure falls back to native compaction. Cancellation and user steering take priority; real overflow keeps native recovery.
• Account for accepted, rejected, and late maintenance usage in session totals, and expose progress/cancellation in the CLI.

Existing explicit off and lightweight-v1 settings remain effective. --context-projection off selects native compaction. Disabling automatic compaction also disables implicit CLM; explicitly selecting clm-v1 still permits manual edits. /compact remains available for a native summary, and /clm-compact requests an explicit working-context edit.

This PR includes the CLM implementation and its tested cache/fallback behavior. The independent completion verifier is outside its scope.

Validation

• Offline workspace build passed.
• pnpm run check passed, including formatting, type checking, repository boundaries, and browser smoke checks.
• ./test.sh passed with its isolated test environment. Coding-agent: 4,344 passed / 55 skipped; CLI: 666 passed; agent-core: 524 passed / 1 skipped; providers: 435 passed / 6 skipped. The remaining workspace and script tests also passed.
• Default-mode integration tests cover first-request setup, automatic edits, reduced working-context estimates, final-answer deferral, explicit disabling, cache-prefix reuse, cancellation, stale edits, and native fallback. Native-only regressions explicitly select off and retain their original assertions.
• Built CLI help reports clm-v1 as the default. A real CLI regression also verifies that explicit --context-projection off survives resource loading and leaves the config file unchanged.