Fix #285: make 1h prompt caching reachable via config

[ericleepi314]

Closes #285

Stacked on #316 (deep stack down to #304). Merge in order; GitHub retargets automatically.

Summary

The 1h prompt-cache path was end-to-end dormant: evaluate_prompt_cache_1h_eligibility had zero production callers (the latch stayed None forever) and prompt_cache_1h_allowlist was never populated — every cache_control emitted ttl: '5m' regardless of model or config.

What's wired

• Config-backed allowlist (the issue's non-GrowthBook channel): new settings.prompt_cache_1h_sources (list, e.g. ["repl_main_thread"]) with CLAWCODEX_PROMPT_CACHE_1H_SOURCES env override (comma-separated; set-but-empty is a documented kill switch; list settings REPLACE across config layers). Installed once per session — sticky, like every field in this module (a mid-session flip would bust the cached prefix).
• Eligibility latch wired: initialize_prompt_cache_state() runs from init.pre_action (env-signal backed: CLAUDE_CODE_USER_TYPE=ant / CLAUDE_CODE_IS_CLAUDE_AI_SUBSCRIBER / CLAUDE_CODE_IS_USING_OVERAGE) and lazily from should_1h_cache_ttl when the latch is unevaluated — exact TS parity (claude.ts:420-425 evaluates at the consumer). The lazy path covers SDK embedders that skip pre_action and a /clear that resets the latch singleton, which would otherwise have silently downgraded the rest of the session to 5m (critic-found gap).
• The TTL decision stays the TS AND-gate: eligibility and source-in-allowlist; everything else stays 5m (the Phase-1 default).

Scope decision (matches the issue's "as the corresponding feature lands")

The three remaining toggle latches (afk_mode / cache_editing / thinking_clear) stay as documented dead stores: their source features (TUI AFK toggle, cache-editing config, extended-thinking request parameter) don't exist in this port, and adding caller-less setters would recreate the exact dead API the original audit flagged. The audit notes now state the precise feature-not-built reasons, and all stale "no production caller / future WI" docstrings were brought in line with the wired reality.

Test plan

• 10 new tests: sticky populate + garbage rejection; env+eligibility activates 1h end-to-end through the real should_1h_cache_ttl; sources-without-eligibility stays 5m (AND-gate); settings-backed sources; empty-env kill switch; no-config dormant; idempotence; 1h recovers after clear_beta_header_latches() (the /clear downgrade fix)
• 133 tests across session_start / cache_state / init / prompt_assembly / settings / clear-sections files pass
• Full suite on the stack: 7888 passed, 0 failed, 5 skipped
• Critic review loop: APPROVE after 1 revision round (the stale-docstring contradiction and the /clear silent-downgrade both came from it; the lazy-consumer fix was verified for singleton-aliasing safety)

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