feat: disable OpenCode compaction and add proactive token budget pruning

[tuanhung303]

Summary

• Disables OpenCode's built-in compaction via the config hook — prevents OpenCode from summarizing and dropping conversation history before the plugin can process it
• Adds proactivePrune strategy that manages token budget at the plugin level: prunes old tool outputs at 70% context usage, adds reasoning block pruning at 85%
• Safety net compacting hook preserves plugin state (active todos, tracked files, pruning stats) in any compaction summary if compaction is ever re-enabled

Problem

OpenCode's auto-compaction runs before the plugin's experimental.chat.messages.transform hook, so when it triggers it drops all history — overriding the plugin's granular pruning. The plugin never sees the messages OpenCode already removed.

Solution

1. Config hook sets compaction: { auto: false, prune: false } so OpenCode never triggers its own compaction
2. proactivePrune strategy replaces OpenCode's role in managing context window pressure with configurable thresholds
3. compacting hook acts as a last-resort safety net if a user re-enables compaction manually

Files Changed

File	Change
index.ts	Config hook + compacting safety net hook
lib/strategies/proactive-prune.ts	NEW — proactive pruning strategy (~235 lines)
lib/strategies/index.ts	Export proactivePrune
lib/hooks.ts	Register proactivePrune in PRUNE_STRATEGIES pipeline
lib/config/schema.ts	TokenBudgetSchema with configurable thresholds
lib/config/defaults.ts	Default token budget values
tests/e2e/proactive-pruning.test.ts	NEW — 13 E2E tests
tests/hooks.test.ts	Mock fix for new strategy
tests/integration/opt-in-defaults.test.ts	Mock fix for new strategy

Test Results

• 32/32 test files pass
• 570/570 tests pass (13 new)
• Build clean, npm link verified