fix(subagents): classify valid overrides as info

[627150795]

Problem

Fixes #672. Valid complete Agent Type overrides are currently counted and surfaced as startup problems even when the replacement definition loads successfully.

Value

Users can distinguish intentional overrides from malformed, unreadable, or capability-invalid definitions while still seeing which source replaced which definition.

Approach

• Mark only successfully validated complete-definition replacements as informational and retain the source and replacement message.
• Count only warning diagnostics as problems; summarize informational notices separately.
• Use info severity when startup diagnostics contain only valid overrides; keep mixed notices at warning severity.
• Preserve built-in → global → trusted-project precedence, tool narrowing, model and effort selection, and fail-closed fallback blocking.

Validation

• Candidate baseline: upstream/main at 3cb2ecf; final candidate HEAD: 78e51bf.
• Passed: node --experimental-strip-types --check for all four changed TypeScript files.
• Passed: git diff --check upstream/main...HEAD.
• Failed: bun run check stopped because the clone has no installed dependencies; the first missing package was acorn.
• Failed: bun run test ran the suite but exited nonzero because dependencies are absent, including @earendil-works/pi-coding-agent, typebox, and undici. The two focused subagent test files also could not load because @earendil-works/pi-coding-agent is absent. No dependencies were installed.
• CI on this HEAD (run 37253606035): Runtime Node 22.19/24/26, all UI and Windows jobs, and Web E2E shard 2/2 passed. The overall run failed because shard 1/2 failed in unchanged tests/web/message-rerun.e2e.ts while expecting the fork success notice; the page instead showed a fork-result uncertainty alert. GitHub denied rerunning failed jobs without repository admin rights.

Impact

Valid overrides now produce informational startup notices; mixed notices remain warnings and include all diagnostics. Tool/context behavior, persisted data, and compatibility are unchanged.