Add journal capture: gitignored .journal/ and /journal command

[bracyw]

Parent

#677

What to build

The capture half of the journal skill: a local, git-free place to jot notes. A gitignored .journal/ folder at the repo root, organized as categories (subfolders) containing entries (note files), with loose root entries also allowed. A /journal command writes an entry; hand-editing files stays supported. This slice stands alone — capturing a note is fully usable before any export logic exists.

Acceptance criteria

• .journal/ is added to .gitignore and journal entries are never tracked; the folder stays separate from .context/ (which holds cloned reference repos)
• /journal writes a timestamped entry into .journal/, into an optional category subfolder when one is given, or the root otherwise
• Capturing touches no git state — no commit, branch, or push
• The workflow terms Journal, journal entry, and category are defined in docs/agents/glossary.md alongside the existing Idea term
• SKILL.md documents capture and notes that hand-editing files in .journal/ is equally valid

Blocked by

None - can start immediately