fix(config): stop config set compression.<lcm_knob> crying wolf

[Kyzcreig]

hermes config set compression.skew_floor 0.55 printed:

⚠ 'compression.skew_floor' is not a recognized config key — it was saved
  anyway, but Hermes may not read it.

Hermes does read it. The LCM context-engine plugin reads several compression.*
keys through its own explicit _hermes_compression_float bridge
(plugins/context_engine/lcm/config.py), but they were never declared in
DEFAULT_CONFIG, so _validate_config_key classified each one as unknown.

Measured 2026-08-09 — 4 of the 5 keys the plugin reads warned falsely:

OK    compression.target_ratio
MISS  compression.skew_floor                    <- warning since it shipped
MISS  compression.calibration_hard_frac
MISS  compression.maintenance_min_pressure_ratio
MISS  compression.maintenance_max_cache_hit_ratio

This is worse than cosmetic. The identical message is the ONLY signal for a
genuinely INERT knob — a key written to config.yaml that the runtime never reads.
That failure mode has shipped three times in this subsystem (#506 was dead until
#508 bridged it). A warning that is wrong by construction trains the operator to
ignore it, so the false positive disarms the real alarm.

Fix: declare the four keys in config_defaults.py's compression block with their
plugin defaults and a comment explaining that the plugin, not the core
compressor, reads them. Values match the plugin's own defaults, so behavior is
unchanged — this is schema declaration, not a new setting.

Deliberately NOT done: adding compression to the open-container escape list.
That would accept anything under compression. and silently swallow real typos.

Tests: 14 in tests/hermes_cli/test_lcm_compression_config_keys.py. The knob list
is DERIVED from the plugin source by regex rather than hardcoded, so a future
bridge entry added without a schema declaration fails immediately instead of
shipping another false warning. Includes a positive control (a regex matching
nothing would make the suite vacuous) and two negative controls: a typo must
still be rejected AND still produce the did-you-mean suggestion, and unrelated
garbage under compression. must stay unknown.

RED-proven: reverting the schema addition fails 9 of 14, including
test_a_typo_is_still_caught_and_suggested — the suggester can only propose the
real key once it is in the schema, so the fix also improves typo diagnostics.

533 passed in tests/hermes_cli -k config. The 5 failures in that selection are
pre-existing: pristine fork/main fails the same set under the same command.

[Kyzcreig]

FleetReview

Confidence: 3/5

Findings

• P1 hermes_cli/config_defaults.py:627 — compression.skew_floor default declared as 0.55 silently lowers the built-in compressor's late-compaction guard from 0.7
• P2 hermes_cli/config_defaults.py:615 — Comment's "NOT by the core compressor" premise is false for skew_floor/calibration_hard_frac, and schema-declaring them makes config_defaults.py the effective source of truth

---

FleetReview provenance · models: B=gpt-5.6-sol, C=claude-code-opus-4-8, F=gpt-5.6-sol, G=grok-4.5 · cost: $4.15 · duration: 11m 27s · rounds: 1 · files examined: 2