Add --tag/--category group selection to skill install and uninstall (#102)

[devrimcavusoglu]

Summary

Fixes #102.

• skill install and skill uninstall accept --tag, --category (repeatable), and --include-untagged — the same filters skill list has. They're defined once as a skillFilter in skill_helpers.go (flags + help + match semantics), and skill list is refactored onto it too, so the three commands can't drift.
• Filters resolve against the registry at --scope (consistent with how install already does reg.Get(name, scopeVal)); resolved names are processed in sorted order and reported per-skill exactly like a name batch.
• Names and filters are mutually exclusive (validation error, exit 2), and passing neither is also a validation error (previously cobra's MinimumNArgs plain error).
• Empty match is an error — no registered skills match --tag X in <scope> scope (exit 1), never a silent no-op.
• uninstall --tag resolves the group from the registry, then intersects with InstalledSkills(scope): tagged-but-not-installed skills are skipped rather than reported as per-skill failures; nothing installed → no installed skills match --tag X on <platform> (<scope> scope).
• --enforce-budget counts the resolved group.

Issue DoD

• --tag on skill install and skill uninstall (plus --category, since it's the same filter object)
• --tag and explicit names are mutually exclusive — documented and enforced
• Empty match is a clear message, not a silent no-op

Test plan

• 9 new tests in platform_test.go: install by tag, tag AND category, category comma-list OR, empty match, names+filter conflict, neither-given, malformed --category, scope-respecting resolution, --enforce-budget with resolved count, uninstall-by-tag skips uninstalled, uninstall nothing-installed error, uninstall conflict
• go test ./..., make lint green
• Manual smoke with the built binary (transcript in the session): install --tag workflow → two installs; --tag nope → exit 1 with message; b1 --tag workflow → exit 2; uninstall --tag workflow → both removed

🤖 Generated with Claude Code

[devrimcavusoglu]

Review (adversarial pass against #102)

DoD: all three items hold (mutual exclusion chosen and enforced consistently in help/docs/CHANGELOG; empty match is exit 1 with a message; skill list is a pure refactor — help text and --json output byte-identical to main).

Findings and what changed in af01d75 / 3fa77bc:

Sev	Finding	Resolution
Medium	TestSkillInstall_Filter_EnforceBudgetCountsResolved only ran the happy path — reverting the fix to len(args) still passed	Test seeds the platform to threshold-1 and asserts the refusal, then fits exactly at threshold
Low/Med	Registry parse warnings were swallowed by install/uninstall --tag (a malformed SKILL.md can be why a tag matches nothing; skill list shows them)	Warnings appended to the empty-match error and printed to stderr otherwise (TestSkillInstall_Filter_SurfacesParseWarnings)
Low	Installed-but-already-removed-from-registry skills are unreachable by uninstall --tag (by design)	Documented: uninstall a group before removing it from the registry
Low	uninstall duplicated the names-path of resolveActionTargets	resolveActionTargets takes a lazy registry constructor; duplication gone
Nit	Roadmap still listed #102; guides didn't show group installs	Fixed

Noted, not changed: no-args install/uninstall now exit 2 (ValidationError) instead of cobra's exit 1 — deliberate, matches the repo's "validation failure" code, called out in the CHANGELOG.

Stacked on #107 (base feature/init-no-instructions); CI runs once the base becomes main.