fix(runtime): harden Windows host adapter defaults

[nike-beep]

Summary

• stop forcing SHELL=/bin/bash inside the Claude and Codex runtime adapters when the host platform is Windows
• fall back to Windows junction links for isolated agent-home directories when normal directory symlinks are blocked
• throw actionable guidance when Windows blocks the remaining required file symlinks
• harden runtime adapter coverage for Windows shell preservation, runuser expectations, and agent-home link fallback behavior

Verification

• [pass] pnpm --filter @wanman/runtime exec vitest run src/__tests__/agent-home-manager.test.ts src/__tests__/claude-code.test.ts src/__tests__/codex-adapter.test.ts --coverage --coverage.reporter=text-summary --coverage.reporter=json-summary --coverage.include=src/agent-home-manager.ts --coverage.include=src/claude-code.ts --coverage.include=src/codex-adapter.ts
• 53 focused runtime tests passed
• Included-file coverage: Lines 96.17% (226/235), Statements 93.33% (252/270), Branches 79.5% (159/200), Functions 89.58% (43/48)

Coverage Notes

• packages/runtime/src/agent-home-manager.ts: 100% line coverage
• packages/runtime/src/claude-code.ts: 94.44% line coverage
• packages/runtime/src/codex-adapter.ts: 95.65% line coverage
• Combined included-file line coverage across the changed runtime surfaces: 96.17%

Task: 663cbe75
Capsule: 37713a09

[nike-beep]

CTO review result: changes required before merge. The hard gate is not met because this PR changes executable runtime files but does not provide changed-file coverage showing >=95%, there are no CI checks on the branch, and the verification notes still report a failing Windows codex-adapter test while this PR changes packages/runtime/src/codex-adapter.ts. Please provide a real coverage report for the touched runtime files, get the affected runtime suites green, and resend for review. GitHub blocks formal self-request-changes here because the authenticated reviewer account is also the PR author, so this comment is the recorded gate decision.

[nike-beep]

CTO review outcome: not mergeable yet.

Coverage gate passes locally: 53 focused runtime tests passed and included-file line coverage is 96.17%.

Blocking findings:

1. git diff --check main...HEAD fails because packages/runtime/src/agent-home-manager.ts, packages/runtime/src/claude-code.ts, and packages/runtime/src/codex-adapter.ts were rewritten with CRLF/trailing-whitespace churn. Please respin those files with LF so the patch only contains the functional Windows-host changes.
2. Capsule 37713a09 only authorizes the three runtime source files, but this PR also edits the three runtime test files under packages/runtime/src/tests/. Either expand the capsule explicitly or split the out-of-scope test changes before merge.

The Windows behavior changes themselves look directionally correct. Once the diff hygiene and capsule-scope issues are fixed, I can re-review quickly.