milestone: integrate M1+M2 changes to unblock merge order

[SupenBysz]

Purpose

This PR provides a single integrated branch containing both M1 and M2 deliverables to avoid merge-order blocking.

Includes

• M1 baseline regression stabilization (equivalent scope of fix: stabilize fallback/notification regressions and add issue delivery docs #2119)
• M2 configurable injection + plugin loader fallback (equivalent scope of feat: add default injection toggle and managed fallback plugin discovery #2122)
• milestone alignment docs synced with GitLab ktai roadmap

Merge Strategy

• Option A (preferred for unblocking): merge this PR directly
• Option B: continue with ordered merge fix: stabilize fallback/notification regressions and add issue delivery docs #2119 -> feat: add default injection toggle and managed fallback plugin discovery #2122

Verification

• bun test src/shared/model-error-classifier.test.ts src/hooks/model-fallback/hook.test.ts src/plugin/event.model-fallback.test.ts src/cli/model-fallback.test.ts src/hooks/session-notification.test.ts src/hooks/session-notification-input-needed.test.ts src/plugin/tool-execute-before-session-notification.test.ts src/cli/mcp-oauth/login.test.ts src/plugin-config.test.ts src/plugin/hooks/create-transform-hooks.test.ts src/features/claude-code-plugin-loader/discovery.test.ts
• bun run typecheck
• bun run build

Result:

• tests: 87 pass / 0 fail
• typecheck: pass
• build: pass

Notes

• Existing PRs fix: stabilize fallback/notification regressions and add issue delivery docs #2119 and feat: add default injection toggle and managed fallback plugin discovery #2122 are still valid.
• This PR is created solely to remove sequencing blockage when maintainer permissions/process require a single merge action.

---

Summary by cubic

Integrates M1 regression stabilization and M2 injection/loader updates into one branch to unblock merge sequencing. Stabilizes model fallback and session notifications, and adds a context-injection toggle plus a managed default plugin when no DB exists.

• New Features

  • Add default_injection_toggle to config and gate the context-injector hook (defaults to on).
  • Inject a managed builtin plugin when installed_plugins.json is missing in the Claude plugin loader; discovery tests added.
  • Add milestone-aligned delivery docs synced with the GitLab ktai roadmap.

• Bug Fixes

  • Make model fallback provider selection deterministic by preferring the current provider when available.
  • Fix session-notification command finder to clear pending state and avoid cross-test leakage; tighten return typing.
  • Ensure spy cleanup in the session-notification tool execution test.

^{Written for commit 8610160. Summary will update on new commits.}

[cubic-dev-ai]

No issues found across 13 files

Confidence score: 5/5

• Automated review surfaced no issues in the provided summaries.
• No files require special attention.

_{Requires human review: Logic changes in model fallback selection and plugin discovery (M2) introduce new behaviors that, while intended to stabilize, carry inherent risk under '100% sure no regressions' criteria.}

[SupenBysz]

Milestone execution status:

• This integrated PR now has all checks green (cla / GitGuardian / cubic).
• It contains both M1 + M2 deliverables and is ready to merge.
• Merge attempt from this account is blocked by repository permission (MergePullRequest denied).

Maintainer action:

1. Merge milestone: integrate M1+M2 changes to unblock merge order #2123 into dev
2. Trigger M3 closure verification on dev using:
   • bun test src/shared/model-error-classifier.test.ts src/hooks/model-fallback/hook.test.ts src/plugin/event.model-fallback.test.ts src/cli/model-fallback.test.ts src/hooks/session-notification.test.ts src/hooks/session-notification-input-needed.test.ts src/plugin/tool-execute-before-session-notification.test.ts src/cli/mcp-oauth/login.test.ts src/plugin-config.test.ts src/plugin/hooks/create-transform-hooks.test.ts src/features/claude-code-plugin-loader/discovery.test.ts
   • bun run typecheck
   • bun run build

I can run and post the M3 closure report immediately after merge.