Fix debounce timer memory leak and suppress timer cleanup

[siddseethepalli]

Summary

• Replace debounceTimers array with a Map<string, Timer> instance field — the Map naturally deduplicates by filename, preventing unbounded growth from repeated file-change events
• Track the suppressConfigReload reset timer in the Map so it gets cancelled on server stop
• Reset suppressConfigReload immediately if saveRawConfig throws, preventing hot-reload from staying permanently disabled after a failed model update
• Remove unused join import

Context

Addresses all 3 review comments from Devin and Codex on #124:

1. Memory leak (Devin): this.debounceTimers.push(timer) accumulated timer refs without cleanup — replaced array with Map instance field that naturally deduplicates
2. Untracked timer (Devin): The 300ms suppressConfigReload reset timer wasn't tracked for cancellation on server stop — now stored in the debounce Map
3. Permanent suppression on failure (Codex): If saveRawConfig threw, suppressConfigReload stayed true forever — now wrapped in try/catch that resets the flag on failure

Test plan

• npx tsc --noEmit passes
• bun test passes (212 tests)
• Manual: verify config hot-reload still works after model set
• Manual: verify daemon stop doesn't leave stale timers

🤖 Generated with Claude Code

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[devin-ai-integration]

Devin Review found 1 potential issue.

View 5 additional findings in Devin Review.

[devin-ai-integration]

🟡 Missing clearTimeout before overwriting __suppress_reset__ timer in debounceTimers map

When handleModelSet is called multiple times in quick succession, the previous __suppress_reset__ timer is overwritten in the map without being cancelled first. The orphaned timer still fires and prematurely resets suppressConfigReload to false, allowing the file watcher to trigger a redundant config reload that evicts sessions.

Root Cause

At assistant/src/daemon/server.ts:377-378, a new timer is created and stored in the map:

const resetTimer = setTimeout(() => { this.suppressConfigReload = false; }, 300);
this.debounceTimers.set('__suppress_reset__', resetTimer);

But unlike the file watcher debounce code at assistant/src/daemon/server.ts:121-122 which properly clears existing timers:

const existing = this.debounceTimers.get(filename);
if (existing) clearTimeout(existing);

…the handleModelSet method does not clear the previous __suppress_reset__ timer before setting a new one.

Scenario:

1. First handleModelSet call: sets suppressConfigReload = true, creates timer A (300ms), stores as __suppress_reset__
2. Second handleModelSet call (within 300ms): sets suppressConfigReload = true, creates timer B (300ms), overwrites timer A in the map
3. Timer A fires (~300ms after first call): sets suppressConfigReload = false prematurely
4. File watcher event from the second write arrives: suppressConfigReload is already false, so it triggers a redundant reload that evicts sessions

Impact: Rapid model changes can cause unexpected session eviction due to the suppression window ending too early.

Suggested change

	const resetTimer = setTimeout(() => { this.suppressConfigReload = false; }, 300);
	this.debounceTimers.set('__suppress_reset__', resetTimer);
	const existingSuppressTimer = this.debounceTimers.get('__suppress_reset__');
	if (existingSuppressTimer) clearTimeout(existingSuppressTimer);
	const resetTimer = setTimeout(() => { this.suppressConfigReload = false; }, 300);
	this.debounceTimers.set('__suppress_reset__', resetTimer);

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