feat(desktop): add trustworthy agent observability

[gagan114662]

Summary

• add an evidence-first session timeline that separates observed actions, inferred explanations, and missing telemetry
• persist causal candidates in SQLite with integrity verification and owner inspection
• add a controlled remediation replay flow with explicit owner approval and replay results
• ingest observer evidence into the causal ledger and surface the workflow in the managed-agent session panel
• default managed agents to one runtime worker to avoid accidental local process exhaustion

Scope

This is the clean observability-only extraction of the earlier mixed branch. It intentionally excludes Numbat/Guardian UI, Open Interpreter, and onboarding changes so each can be reviewed independently.

Verification

• complete Desktop unit suite
• Desktop TypeScript type check
• Desktop repository checker (Biome, file-size ratchet, text-size and pubkey rules)
• Rust formatting check
• git diff whitespace check

The full Rust compile/test suite was not rerun locally because this checkout previously exhausted the available disk during a fresh Tauri build; CI is the authoritative Rust build gate.

Review notes

The branch contains 22 files and approximately 3.5k added lines, reduced from the mixed branch's 91 files and approximately 10k changed lines.