feat: add changelog-observer skill for subagent context (closes #1) (#7)

- SKILL.md defines event tracking (file-change, test-result, decision)
- Logs to .changelog/session-YYYY-MM-DD.md
- Includes template.md for entry structure

Co-authored-by: OpenAI Codex <noreply@openai.com>

Author: gn00295120

.claude/skills/changelog-observer/SKILL.md

@@ -0,0 +1,38 @@
+---
+name: changelog-observer
+description: Track development session events in a daily markdown changelog, including file changes, test results, and key decisions.
+allowed-tools:
+  - Read
+  - Write
+  - Edit
+  - Bash
+user-invocable: true
+---
+
+# Changelog Observer
+
+Track session context in `.changelog/session-YYYY-MM-DD.md`.
+
+## Log target
+
+- Use `.changelog/session-YYYY-MM-DD.md` for the current local date.
+- Create `.changelog/` and the session file if they do not exist.
+- If the session file is new, initialize it from [template.md](template.md).
+- Append new entries in chronological order.
+
+## What to log
+
+- `file-change`: after creating, editing, or deleting project files.
+- `test-result`: after running tests, checks, or validation commands.
+- `decision`: after making an implementation or architecture decision worth preserving.
+
+## Entry rules
+
+- Keep each description short and specific.
+- List only directly affected files.
+- Use `none` when no files were changed.
+- Do not rewrite earlier entries unless they are incorrect.
+
+## Template
+
+Use the markdown structure in [template.md](template.md) for each appended entry.

.claude/skills/changelog-observer/template.md

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+# Session Changelog - YYYY-MM-DD
+
+### [timestamp]
+- Event Type: [file-change | test-result | decision]
+- Description: [short summary]
+- Files Affected: [path/to/file, another/path] | none