feat(subagents): builtin override documents retune shipped delegates

[shabhui]

What happened

The five shipped builtin subagents (explorer, code-reviewer, test-runner, fixer, ui-designer) could only be switched on or off (ADR 0270). A user who wanted test-runner pinned to a cheaper model, or fixer with a stricter write policy, had no supported path: copying the builtin into ~/.agents/subagents creates a second Settings row that claims the same handle, and the two enablement switches then disagree.

What this adds

An app-owned override directory, <data>/subagent-overrides/*.md, whose Markdown documents retune a shipped builtin by name (ADR 0319):

• an override replaces the named builtin's definition in the catalog; it never adds a delegate or a Settings row — a name no builtin uses is a load diagnostic, not a new delegate;
• overrides parse as builtin source, so the ADR 0270 Settings switch still governs the retuned definition and Settings renders one row;
• user documents keep outranking overrides, which keep outranking the shipped constants;
• the directory is re-read on every session launch and every subagent/catalog request, so an edit reaches every session — open ones included — on its next prompt;
• Electron main wires builtinOverridesDir(dataDir); the loader stays directory-agnostic and testable.

Example: a <data>/subagent-overrides/test-runner.md with a different model: pin changes which model the shipped test-runner delegate uses, with no second row anywhere.

Docs

• new ADR 0319 (context, decision, alternatives), ADR 0062 status line + §1 amendment, ADR README index
• 03-runtime/02-agent-runtime.md §5f catalog paragraph (en + zh)
• 04-ux/06-settings-ia.md Built-in group note (en + zh)
• 06-delivery/04-e2e-test-plan.md E2E-1175 journey

Verification (Windows 11, Node 24, Pi 1.0.1 baseline = current main)

• agent-runtime vitest subagent-definitions + subagent: 65/65 pass (25 new override cases: merge order, unknown-name diagnostic, user-document precedence, disabled-builtin interplay, re-read per launch)
• desktop node:test subagent-builtin-overrides.test.mjs: pass (session-launch wiring reads <data>/subagent-overrides)
• biome check on all six touched source files: pass
• tsc --noEmit agent-runtime + desktop: pass

App version: main @ c79b6ca
OS: Windows 11 26220.9587