feat: add update skill for safe cc10x upgrades

[IdoGil-boop]

Summary

Adds a new update skill (plugins/cc10x/skills/update/SKILL.md) that automates the cc10x update cycle while preserving local modifications to cached skill files.

Problem

Users who customize cached skill files (especially cc10x-router/SKILL.md) lose their modifications every time the plugin updates, since the update overwrites the cache with fresh files from the marketplace repo.

Solution

A 5-phase workflow:

1. Discovery — Reads installed_plugins.json + marketplace.json, fetches upstream, displays version status table + changelog preview
2. Stash — Diffs every cached file against the marketplace source (before git pull), saves unified patches + full backups to a timestamped directory
3. Pull & Rebuild — git pull --ff-only, copies plugin files to new cache directory, updates registry JSON via python3
4. Rebase — patch --dry-run each saved diff; applies clean patches automatically, asks user per conflict via AskUserQuestion
5. Verify — Validates cache structure + registry consistency, displays before/after summary table

Design decisions

Choice	Rationale
diff/patch over git stash	Cache directory isn't a git repo
python3 for JSON mutation	Portable — no jq dependency (requires Homebrew on macOS)
AskUserQuestion at 3 gates only	Proceed? / Old cache cleanup? / Per-conflict resolution
Move, never delete	Old caches go to timestamped backup dir, never rm -rf
Diff before pull	Isolates user changes from upstream changes

Triggers

update cc10x, upgrade cc10x, pull cc10x, sync plugin, refresh cc10x, check for updates

Test plan

• Run skill when already up-to-date — should detect and offer re-sync
• Manually modify a cached file, then run skill — should stash diff, pull, and rebase patch
• Introduce a conflict (modify same lines as upstream) — should ask user per conflict
• Verify installed_plugins.json is correctly updated after run
• Verify backup directory contains patches + originals

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