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.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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/implement/SKILL.md

@@ -0,0 +1,15 @@
+---
+name: implement
+description: "Implement a piece of work based on a PRD or set of issues."
+disable-model-invocation: true
+---
+
+Implement the work described by the user in the PRD or issues.
+
+Use /tdd where possible, at pre-agreed seams.
+
+Run typechecking regularly, single test files regularly, and the full test suite once at the end.
+
+Once done, use /code-review to review the work.
+
+Commit your work to the current branch.