feat(scheduler): add cache_ttl "never" sentinel for always-warm lanes

[iceteaSA]

Problem

cache_ttl assumes idle > TTL means the provider evicted the prompt cache, so the next prefix rebuild is free. Two consumers act on that assumption:

1. the scheduler converts a defer pass into an execute pass (scheduler.ts), and
2. mustMaterialize fires the ttl_idle HARD fold (via hardCacheExpired in transform.ts).

On lanes kept warm by an external keepwarm mechanism (prewarm proxies, dedicated cache-keep tools that re-warm the provider cache out-of-band), the assumption is wrong: the cache never goes cold, MC's lastResponseTime goes stale anyway, and both consumers false-positive. MC then initiates a rebuild it believes is free that is actually a full paid cache-write — measured 450-560K tokens per fold on large sessions. There was no way to express "this lane never goes cold": the config requires a duration.

Fix

cache_ttl: "never" (case-insensitive, works as the string form or any per-model value):

• parseCacheTtl("never") returns Infinity; both consumers go inert through the existing comparisons (elapsed > Infinity / elapsed >= Infinity are never true) — no new branches in the hot path.
• The Rust scheduler mirrors the sentinel with u64::MAX (its predicates already use saturating_sub, so no overflow path).
• Status surfaces render honestly instead of a bogus countdown: /ctx-status shows never expires (always-warm lane); the TUI sidebar gets a JSON-safe cacheNeverExpires flag on StatusDetail (Infinity does not survive JSON-RPC); Pi's status dialog now uses the shared parseCacheTtl (it previously used a private fallback parser that would have shown a 5m countdown).
• hardCacheExpired is extracted into a pure computeHardCacheExpired helper so the "never" -> Infinity -> false chain has direct unit coverage instead of only flag-consumption coverage.

Docs: CONFIGURATION.md describes the sentinel, what it disables, and the tradeoff — after a genuinely cold start (e.g. the keepwarm process died), the free-fold window is not detected on such lanes; mutations then apply at the execute threshold.

Verification

• plugin: 2995 pass / 0 fail; pi-plugin: 666 pass / 0 fail; tsc --noEmit clean on both
• new tests red-verified: reverting the sentinel fails the scheduler tests and the computeHardCacheExpired test
• Rust: sentinel + predicate + scheduler-decision tests added in scheduler.rs's test module (note: I could not run cargo test locally — the crates workspace has path deps outside this repo; the change is a 3-line early return mirroring the TS logic)
• schema + generated docs regenerated (build-schema / build-config-docs drift tests green); check:tui-compiled green

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Summary by cubic

Adds cache_ttl: "never" to disable the idle‑TTL heuristic on always‑warm lanes, preventing false executes and expensive cache rebuilds. Also fixes status serialization and restores invalid‑TTL diagnostics.

• New Features

  • parseCacheTtl("never") returns Infinity (TS) / u64::MAX (Rust) so idle‑execute and ttl_idle never fire; computeHardCacheExpired exported for consistent checks with onInvalid.
  • Status UIs show “never expires” using a JSON‑safe cacheNeverExpires flag; Pi now uses the shared parser; docs and schema updated.

• Bug Fixes

  • Status RPC keeps cacheRemainingMs numeric (0) when TTL is “never” and keys on cacheNeverExpires.
  • Transform restores invalid‑TTL outcomes and session logs via the onInvalid callback.

^{Written for commit 222c184. Summary will update on new commits.}

Greptile Summary

Adds and consistently surfaces the always-warm cache_ttl: "never" configuration.

• Maps the sentinel to non-expiring TTL values in the TypeScript and Rust schedulers.
• Prevents idle-TTL scheduling and hard-fold triggers for always-warm lanes.
• Preserves invalid-TTL diagnostics in the extracted hard-expiry helper.
• Adds a JSON-safe RPC flag and updates status interfaces, tests, schema, and documentation.

Confidence Score: 5/5

The PR appears safe to merge.

No blocking failures remain.

Important Files Changed

Filename	Overview
packages/plugin/src/plugin/rpc-handlers.ts	Represents non-expiring cache status with a JSON-safe boolean while keeping numeric RPC fields finite.
packages/plugin/src/hooks/magic-context/transform.ts	Extracts hard cache-expiry computation, supports the non-expiring sentinel, and preserves invalid-TTL diagnostics.
packages/plugin/src/features/magic-context/scheduler.ts	Parses the case-insensitive sentinel as an infinite TTL so existing comparisons suppress idle execution.
crates/mc-module/src/scheduler.rs	Mirrors non-expiring TTL semantics with u64::MAX and adds predicate and scheduler coverage.
packages/plugin/src/shared/rpc-types.ts	Extends status details with the optional JSON-safe non-expiring-cache indicator.
packages/pi-plugin/src/dialogs/status-dialog.ts	Uses shared TTL parsing and renders always-warm lanes without a misleading countdown.

_{Reviews (3): Last reviewed commit: "fix(scheduler): keep cacheRemainingMs JS..." | Re-trigger Greptile}

[cubic-dev-ai]

All reported issues were addressed across 20 files

<sub>Reply with feedback, questions, or to request a fix.

Re-trigger cubic</sub>

[iceteaSA]

Note on the red Check (dashboard): it's pre-existing on master, not introduced here — this PR touches no dashboard files. The same 2 ConfigEditor ⇄ schema parity failures reproduce on a pristine upstream/master checkout at v0.33.0 (48ab531d):

- []
+ [
+   "experimental.mural.enabled",
+   "experimental.mural.model",
+   "fail_closed_blocking",
+   "pi.subagent_extensions",
+ ]

The v0.33.0 schema added those four leaves without updating the dashboard's RENDERED_PREFIXES/OMITTED_BY_DESIGN lists, and experimental.mural.* also trips the "nothing re-introduces the dead experimental.* namespace" test. Happy to send a separate fix PR for the parity lists if useful — the right classification (render vs omit-by-design, and whether mural should live under experimental.) is a maintainer call.