Add support for session/fork

[julianschiavo]

• Adds support for session/fork (unstable, supported by other implementations like claude-agent-acp)
• Advertises the fork capability
• Refactors session state installation to share code between new, forked, and resumed sessions
• Adds tests

This PR description was written by a human. The PR was written by Codex and reviewed by a human.

[fireblue]

Consumer validation from Chat Engine Desktop:

• PR head 849573ed206f7c2851d7ca8f5a50eccf0bf48011 cherry-picked cleanly onto both the published v1.1.9 tag and upstream main efa3789.
• On both bases, npm run typecheck passed.
• On both bases, the full Vitest suite passed: 39 files passed / 6 skipped; 345 tests passed / 28 skipped.
• I also integrated the built result into an ACP desktop client and verified the advertised session/fork capability, thread/fork request path, child session-state installation, registration, and partial-failure cleanup.

This PR matches the native bridge we need; I do not see a reason to open a competing implementation.