docs: document the agent skills system

.agentd/skills/agent-memory/SKILL.md

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+---
+name: agent-memory
+description: Store, search, and manage shared knowledge through the agentd memory service. Use for persisting information across agent sessions and inter-agent knowledge sharing.
+---
+
+# Agent Memory
+
+Skill for interacting with the agentd memory service — a vector-backed knowledge store for agents.
+
+## Storing Memories
+
+```bash
+# Store a fact (default type: information, default visibility: public)
+agent memory remember "The deployment key is in 1Password vault 'Infrastructure'" \
+  --created-by worker --tags deploy,keys
+
+# Store a question
+agent memory remember "Should we migrate to async Redis?" \
+  --type question --tags redis,architecture
+
+# Store a request
+agent memory remember "Need someone to review auth middleware rewrite" \
+  --type request --tags review,auth
+```
+
+## Searching Memories
+
+```bash
+# Basic semantic search
+agent memory search "deployment procedures" --as-actor worker
+
+# Filter by tags
+agent memory search "auth" --tags auth --type information --limit 5
+
+# JSON output
+agent memory search "redis" --json
+```
+
+## What to Remember
+
+- Architectural decisions and their rationale
+- Patterns that worked or failed for specific areas
+- Blockers or gotchas discovered during implementation
+- Conventions not captured in CLAUDE.md
+
+## What NOT to Remember
+
+- Information already in code, comments, or git history
+- Ephemeral task state (use conversation context for that)

.agentd/skills/example/SKILL.md

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+---
+name: example
+description: A minimal example skill demonstrating the skill file format. Copy and adapt this template to create your own skills.
+---
+
+# Example Skill
+
+This is a minimal example skill. Replace this content with instructions or
+reference material for the agent.
+
+## What Skills Are For
+
+Skills are Markdown files that are injected into an agent's `.claude/skills/`
+directory at spawn time. Claude Code discovers and can invoke them via the
+`/skill` command.
+
+## Skill Format
+
+A skill file consists of:
+1. An optional YAML frontmatter block (between `---` delimiters)
+2. Markdown content with instructions, examples, or reference material
+
+### Frontmatter Fields
+
+- `name` — override the skill name (defaults to directory/filename stem)
+- `description` — shown in `agent skill list` output
+
+## Tips
+
+- Write skills as instructions to the agent, not documentation for humans
+- Use code blocks for commands the agent should run
+- Keep skills focused on a single domain or tool
+- Use `> [!IMPORTANT]` callouts for critical constraints

.agentd/skills/git-spice/SKILL.md

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+---
+name: git-spice
+description: Branch stacking and PR management with git-spice. Use when creating stacked branches, submitting PRs, syncing with upstream, or navigating a branch stack.
+---
+
+# git-spice
+
+Reference skill for using git-spice (`git-spice`) in the agentd project.
+
+> [!IMPORTANT]
+> **All agent usage MUST include `--no-prompt`** to prevent blocking on interactive input.
+
+## Common Workflows
+
+```bash
+# Create a new stacked branch
+git-spice branch create issue-NNN -m "feat: description"
+
+# Commit with auto-restack of dependents
+git-spice commit create -m "feat(scope): description"
+
+# Submit a PR
+git-spice branch submit --no-prompt --label review-agent \
+  --title "feat: <title>" \
+  --body "<summary>\n\nCloses #N"
+
+# Sync with upstream
+git-spice repo sync
+
+# Check stack state
+git-spice log short
+```
+
+## Branch Naming
+
+- Implementation: `issue-NNN`
+- Documentation: `docs/issue-NNN`
+- Refactoring: `refactor/issue-NNN`
+
+## Notes
+
+- Never use raw `git checkout -b` — use `git-spice branch create`
+- Never use `gh pr create` directly — use `git-spice branch submit`
+- If TLS errors occur, fall back to `git push origin <branch>` + `gh pr create --base <base> --head <branch>`