feat(agent): add deterministic base-agent orchestrator

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Summary

• adds the deterministic Gittensory base-agent run model, planner, private APIs, and MCP tools
• adds public-safe @gittensory GitHub mention commands for installed repos
• bumps the MCP wrapper to 0.2.0 and updates root/package changelogs

What changed

• Added agent_runs, agent_actions, and agent_context_snapshots storage plus D1 migration.
• Added canonical agent APIs for run creation, next-work planning, branch preflight, blocker explanation, and PR packet generation.
• Added remote MCP tools and local MCP CLI commands under gittensory-mcp agent ....
• Added explicit issue_comment app health requirements and sanitized GitHub mention command handling.
• Updated docs, OpenAPI schemas, changelogs, and tests for the base-agent pivot.

Why

Gittensory now has a single deterministic orchestration layer that can rank, explain, preflight, and draft public-safe packets without becoming an autonomous PR bot. MCP remains the first-class private surface, while GitHub comments stay opt-in, command-driven, and sanitized.

Validation

• npm run test:ci
• Coverage: 247 tests passing, 95.07% branch, 98.24% statements, 99.39% lines
• MCP package dry-run passed with the expected tarball allowlist
• Docs build and docs smoke passed

Notes

• The MCP package version is bumped to 0.2.0, but npm publishing should still happen through the release/tag path after this merges.
• The GitHub App now needs the issue_comment event enabled for mention commands.