feat(subagents): accept @server.tool / server.tool / bare id in subagent tools[]

[Minitour]

Summary

Follow-up to #118 and #119.

Until now subagents[].tools required the bare local tool id, while skills[].def.requires used the @-prefixed qualified form:

# skills.requires — qualified, with @
requires:
  - '@dbx.sql_read_only'

# subagents.tools — bare local id only
tools:
  - sql_read_only

Users who learned requires first wrote @dbx.sql_read_only under a subagent, hit the new unknown-tool warning from #119, and concluded capa is broken — when in fact they'd just used the wrong dialect. The warning made the asymmetry more visible, not less.

This PR adds a resolver that accepts all three equivalent forms for the same tool and routes the three consumers through it:

subagents:
  - id: data-analyst
    tools:
      - sql_read_only           # bare local id (existing form)
      - dbx.sql_read_only       # qualified
      - '@dbx.sql_read_only'    # @-prefixed (requires-style)

All three resolve to the same Tool. Pick whichever reads best in context.

Changes

• src/types/capabilities.ts — new resolveSubagentToolRef(ref, tools) next to getQualifiedToolName. Tries qualified-name match first (handles @server.tool / server.tool / ungrouped command tools), then falls back to bare-id match.
• src/server/mcp-handler.ts:260 — filtered MCP endpoint uses the resolver instead of tools.find(t => t.id === toolId), so subagents with @-prefixed refs can actually call their tools.
• src/shared/providers/handlers.ts (resolveTool) — renderer uses the resolver so all three input forms produce the same canonical <group>.<tool> bullet.
• src/cli/commands/install-tasks/helpers/tool-warnings.ts (collectSubagentRefWarnings) — validator uses the resolver and the warning copy now lists the three accepted forms.
• skills/capabilities-manager/references/capabilities-schema.md — documents all three forms with examples and notes the install-time validation.

Tests

• src/types/__tests__/resolveSubagentToolRef.test.ts (new) — 7 cases covering MCP tool, grouped/ungrouped command tools, and unresolved refs in all three input shapes.
• src/cli/commands/install-tasks/helpers/__tests__/tool-warnings.test.ts — new cases: mixed-form happy path produces no warnings; @-prefixed ref that doesn't resolve still warns with the original ref intact.

Test plan

• bun test — 1095/1095 pass (8 new)
• Built the binary and verified end-to-end against a real project: sql_read_only, dbx.poll_sql_result, and @dbx.sql all rendered the same canonical bullet; only an unresolvable @dbx.does_not_exist produced a warning
• Reviewer: skim the schema doc wording in capabilities-manager — that's the surface external users learn from