feat: add contextManagement.compactionTriggerAt to lower the auto-compaction trigger

[yujiachen-y]

Change

Adds contextManagement.compactionTriggerAt to config.yaml so users can make automatic context compaction start earlier, for example to keep requests smaller. Refs #488.

contextManagement:
  compactionTriggerAt: 0.8 # ratio of the context window, or an absolute token count such as 300000

• A value in (0, 1] is a ratio of the current model's context window; an integer above 1 is an absolute token count.
• The effective trigger is min(setting, built-in trigger). The setting can only start compaction earlier, never later, so it cannot push requests past the Provider input limit. Comparable to Codex's model_auto_compact_token_limit, which is clamped to 90% of the window.
• No lower bound by design: the user owns the choice. The docs state that a value below the post-compaction size compacts before every request.
• Invalid numbers (1.5, 0, negatives, non-finite) are logged once as context_compaction_trigger_config_ignored and treated as unset. Non-number YAML values are dropped by the config parser. A failing config read falls back to the built-in trigger instead of throwing.
• Unset keeps the current behavior exactly.

Out of scope, intentionally: disabling automatic compaction, per-model settings (the ratio already scales with each model's window), and the tool-result watermark (toolResultCompaction). Showing the effective trigger in /context needs the trigger threaded through the context-usage snapshot and is left for a follow-up.

Implementation:

• @mavis/context-manager: resolveCompactionTokenBudget takes an optional compactionTriggerAt; validation lives in isValidCompactionTriggerSetting / resolveConfiguredCompactionTrigger. Only automaticTriggerAt changes; providerInputLimit and its other consumers (manual compaction, reminder admission) are untouched.
• local-runtime-v2: AutomaticContextCompactor reads the setting per call through an injected getter wired in production-composition.ts.
• @mavis/config: parses the new key; it only keeps numbers because @mavis/config cannot depend on @mavis/context-manager.
• Docs: new "Automatic context compaction trigger" section in docs/tui-capabilities.md.

Validation

• Checks run and results: pnpm verify (full profile) on 2a8d121, darwin: 15 gates passed; test:windows and test:release-package not applicable locally.
• New tests: provider-budget.test.ts (ratio, absolute, above built-in, invalid values), automatic-context-compactor.test.ts (probe starts earlier only from a valid setting; invalid or throwing getter keeps the built-in trigger), packages/config/test/context-management-config.test.ts (parsing). The compactor test fails on the baseline compactor and passes with this change.
• Sandbox check (temporary data dir, real config.yaml, 200K window / 32K output): 0.5 → 100,000; 120000 → 120,000; 0.95, 1.5 and "0.5" → built-in 159,600.
• Performance: basic. Unset configuration does not change any runtime path; no history construction, serialization or token-estimation changes.
• NOT RUN: a live long session against a real model; Linux/Windows locally (covered by CI).

Publication and contribution checks

• I have permission to contribute these changes under the existing licenses applicable to the changed files/packages; imported material and its provenance are identified and existing notices are preserved.
• No credentials, account data, real user content, internal source history or private review material is included.
• Added/removed source files were reviewed before regenerating release/public-source.json; new tests are declared in test/vitest-suites.json where applicable.
• Shared English/Chinese documentation and capability/verification records are updated where applicable. Mock/offline results are not described as live-service acceptance.

Maintainer handoff

Publication scope or license changes (if any): none; one new test file.

Shared-source port: pending.

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[hetaoBackend]

LGTM