feat(skills): add personal and productivity agent skills

Add new skill definitions organized by category:

Personal:
- conversation-summary: summarize AI conversations into Obsidian notes
- knowledge-gardener: vault-aware semantic search and Zettelkasten workflows

Productivity:
- grill-me: alias for running a grilling session
- grilling: relentless interview to stress-test plans
- handoff: compact conversation into handoff document for agent continuity
- writing-great-skills: reference guide for skill authoring best practices
- glossary: domain model for skill-building terminology

common/productivity/handoff/SKILL.md

@@ -0,0 +1,16 @@
+---
+name: handoff
+description: Compact the current conversation into a handoff document for another agent to pick up.
+argument-hint: "What will the next session be used for?"
+disable-model-invocation: true
+---
+
+Write a handoff document summarising the current conversation so a fresh agent can continue the work. Save to the temporary directory of the user's OS - not the current workspace.
+
+Include a "suggested skills" section in the document, which suggests skills that the agent should invoke.
+
+Do not duplicate content already captured in other artifacts (PRDs, plans, ADRs, issues, commits, diffs). Reference them by path or URL instead.
+
+Redact any sensitive information, such as API keys, passwords, or personally identifiable information.
+
+If the user passed arguments, treat them as a description of what the next session will focus on and tailor the doc accordingly.