refactor(agents): migrate task logs from CSV to directory-based format

Replace shared CSV files (AGENT_TASK_LOG.csv, WEEKLY_AGENT_TASK_LOG.csv)
with per-entry files in docs/agents/task-logs/{daily,weekly}/. Each log
entry is now a separate file named YYYY-MM-DD_HH-MM-SS_<agent>_<status>.md,
eliminating git merge conflicts and race conditions when multiple agents
run concurrently.

Updated files:
- daily-scheduler.md: rewritten for directory-based log format
- weekly-scheduler.md: rewritten for directory-based log format
- META_PROMPTS.md: updated pre-flight scan and claiming instructions
- AGENTS.md: updated scheduler reference from CSV to directory-based

Migrated all existing CSV entries to individual files in the new
directory structure.

https://claude.ai/code/session_01P62EWUXwupgxPhpnRejUuz

docs/agents/task-logs/weekly/2026-02-12_00-00-00_bug-reproducer-agent_completed.md

@@ -0,0 +1,8 @@
+# Agent Task Log Entry
+
+- **Date:** 2026-02-12
+- **Agent:** bug-reproducer-agent
+- **Prompt:** bitvid-bug-reproducer-agent.md
+- **Status:** completed
+- **Branch:** unstable
+- **Summary:** Created reproducer for hashtag preferences flakiness (tests/hashtag-preferences.test.mjs)

docs/agents/task-logs/weekly/2026-02-12_00-00-01_changelog-agent_completed.md

@@ -0,0 +1,8 @@
+# Agent Task Log Entry
+
+- **Date:** 2026-02-12
+- **Agent:** changelog-agent
+- **Prompt:** bitvid-changelog-agent.md
+- **Status:** completed
+- **Branch:** unstable
+- **Summary:** Drafted changelog and release notes for week of 2026-02-12

docs/agents/task-logs/weekly/2026-02-13_00-00-00_dead-code-agent_completed.md

@@ -0,0 +1,8 @@
+# Agent Task Log Entry
+
+- **Date:** 2026-02-13
+- **Agent:** dead-code-agent
+- **Prompt:** bitvid-dead-code-agent.md
+- **Status:** completed
+- **Branch:** weekly/dead-code-2026-02-13
+- **Summary:** No dead code found