refactor: extract tmux-launch-agent from implement-issue/setup-skills, overhaul PKM skills

- New common/misc/tmux-launch-agent skill with detect-agent script
  and agents-seed.md (moved from engineering/setup-skills)
- implement-issue simplified: delegates agent dispatch to
  tmux-launch-agent instead of inlining it
- setup-skills stripped of Section E (agent CLI config)
- New in-progress/agent-handoff skill using tmux-launch-agent
- PKM skills simplified: conversation-summary and pkm-curation
  rewritten with leaner process, crit streamlined
- knowledge-gardener removed (moved to common/in-progress/)
- All READMEs updated to match

common/in-progress/agent-handoff/SKILL.md

@@ -0,0 +1,53 @@
+---
+name: agent-handoff
+description: Hand the current conversation off to a fresh background agent that picks up the work immediately.
+argument-hint: "What will the next session be used for?"
+disable-model-invocation: true
+---
+
+## Invocation
+
+```
+/skill:agent-handoff [focus description]
+```
+
+Arguments are optional. If provided, they describe what the next session should focus on and the summary is tailored accordingly.
+
+## Process
+
+### 1. Assess the conversation
+
+Review what has been done, what remains, and any user-provided focus. Identify existing artifacts that capture the work so far (PRDs, plans, ADRs, issues, commits, diffs) so they can be referenced rather than duplicated.
+
+If the user passed arguments, treat them as the priority or scope for the next session.
+
+Completion criterion: The conversation is assessed — what's done, what's left, and the user's focus (if any) are identified.
+
+### 2. Compose the handoff summary
+
+Write a summary of the current state so a fresh agent can continue the work without reading the full conversation. Organise it with these sections:
+
+- **Current state** — what was accomplished, branch/commit, what's working
+- **Next steps** — what needs to be done next, in priority order
+- **Open questions** — decisions still needed, unknowns, trade-offs
+- **Suggested skills** — a bullet list of skills the next agent should invoke (e.g. `/tdd`, `/code-review`)
+- **References** — paths or URLs to existing artifacts (PRDs, plans, ADRs, issues, commits, diffs). Do **not** duplicate their content — reference them.
+
+The summary is a **compass**, not a copy: it points the next agent where to go, it does not replay where you've been. Any content already captured in the referenced artifacts does not belong here.
+
+Redact any sensitive information: API keys, passwords, personally identifiable information.
+
+Completion criterion: The summary covers all required sections, references existing artifacts without duplicating them, and contains no sensitive information.
+
+### 3. Launch the background agent
+
+Use the [`tmux-launch-agent`](../misc/tmux-launch-agent/SKILL.md) skill to fork a fresh agent seeded with the summary:
+
+```
+/skill:tmux-launch-agent --name "<descriptive-title>" <handoff-summary>
+```
+
+- `--name` is **required** — use a short descriptive title (e.g. `"Fix login bug"`, `"Implement user roles"`). This sets the display name in the job list, session picker, and terminal title.
+- The handoff summary from step 2 becomes the new agent's initial prompt.
+
+Completion criterion: `tmux new-window` exits 0 and a new tmux window appears with the handoff summary as the agent's prompt.