feat: add IcodeMate agent support

[LeonidasLux]

Add support for IcodeMate, a kilo-based agent tool that stores sessions using
the shared OpenCode format at ~/.local/share/icodemate.

IcodeMate uses the same on-disk layout as MiMoCode (storage/session_diff
for file-backed storage + icodemate.db for SQLite fallback), so the parser
delegates to the existing OpenCode parser and relabels session IDs with the
icodemate: prefix — the same pattern used by Kilo and MiMoCode.

Key changes:

• Register AgentIcodemate type and its AgentDef in the registry
• Add icodemate.go parser with session ID relabeling logic
• Register the icodemateFmt OpenCode format variant in discovery.go
• Wire up sync, discovery, and file-watching in engine.go

Reviewer's attention:

• internal/parser/icodemate.go — the parser wrapper
• internal/parser/discovery.go — the icodemateFmt format definition
• internal/sync/engine.go — sync integration points

Closes #858

[roborev-ci]

roborev: Combined Review (28c55f1)

Medium issue found: parse-diff coverage for SQLite-backed IcodeMate sessions is incomplete.

Medium

• internal/sync/parsediff.go:288 - parse-diff accepts icodemate as a file-backed agent, but does not synthesize SQLite-backed IcodeMate roots like it does for OpenCode/Kilo/MiMoCode. For --agent icodemate on an icodemate.db-only or hybrid root, DB sessions may be skipped instead of re-parsed.

  Suggested fix: add parser.AgentIcodemate to the OpenCode-format DB source case, and add parser.ParseIcodemateSQLiteVirtualPath to stripVirtualSourceSuffix so DB-backed IcodeMate rows and per-session DB parse errors are attributed to icodemate.db.

---

Panel: ci_default_security | Synthesis: codex, 8s | Members: codex_default (codex/default, done, 5m13s), codex_security (codex/security, done, 1m7s) | Total: 6m28s

[roborev-ci]

roborev: Combined Review (b3bdf34)

No issues found.

---

Panel: ci_default_security | Synthesis: codex | Members: codex_default (codex/default, done, 5m51s), codex_security (codex/security, done, 13s) | Total: 6m4s

[LeonidasLux]

@wesm Please review this PR; if there are any issues, feel free to comment and I will make timely revisions.