chore: add shared agent skills (#20)

Adds the shared agent-skill core under `.agents/skills/` (grilling, grill-me, domain-modeling, codebase-design, code-review, diagnosing-bugs, tdd, research, implement, handoff), with `.claude` symlinked to `.agents` so every agent harness picks them up.

Project-local skills are left untouched. The runtime `scheduled_tasks.lock` is deliberately not committed.

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Reviewed-on: #20
Reviewed-by: xtul <mpiasecki720@protonmail.com>
Co-authored-by: vickydotbat <vickydotbat@tutamail.com>
Co-committed-by: vickydotbat <vickydotbat@tutamail.com>

.agents/skills/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.