feat: add oh-my-pi (omp) as a supported harness

[ludwigku]

Issue: Closes #235

What

• Add omp (oh-my-pi) as a supported inner harness, alongside pi — omac start/continue/resume/serve omp now work.
• Ship an omp bridge (.omp/extensions/omac-bridge.ts) that surfaces the skills manifest + sandbox briefing into omp's system prompt.

Why

omp is a coding-focused fork of Pi with a diverged extension surface, a different discovery root and event/handler shapes. The existing pi descriptor and .pi/extensions/omac-bridge.ts don't cover it.

How

• discovery root .omp (omp does not scan .pi/extensions);
• cwd read from the handler ctx (omp's events carry no cwd/directory);
• before_agent_start returns systemPrompt as a string[]; the fetch is bounded (AbortSignal.timeout) to fit omp's 30 s per-handler cap.
• Targets omp's extension subsystem (not the hooks/ subsystem, whose before_agent_start only allows { message }), preserving the single-system-block invariant via setTurnSystemPromptOverride → agent.setSystemPrompt.
• Tests: TestOmpHarnessDescriptor + path/session/sandbox derivations (internal/config/harness_test.go); TestOmpBridgeInjectsExactlyOneSystemBlock
  (internal/bridge/bridge_test.go) guarding the single-system-block invariant on the omp bridge.
• Docs: HARNESSES.md, MULTI_DIR_DESKTOP.md, README.md, INSTALLATION.md (launch/resume/bridge/skills tables).

Verification

  $ go build ./...                                          # OK
  $ gofmt -l internal/config/harness.go                     # (no output — formatted)
  $ go test ./internal/config/... ./internal/bridge/... ./internal/session/...
  ok  github.com/tngtech/oh-my-agentic-coder/internal/config
  ok  github.com/tngtech/oh-my-agentic-coder/internal/bridge
  ok  github.com/tngtech/oh-my-agentic-coder/internal/session
  $ go test ./internal/config/... ./internal/bridge/... -run Omp -v
  --- PASS: TestOmpHarnessDescriptor / TestOmpConfigHome / TestOmpConfigHomeEnvOverride
  --- PASS: TestOmpGlobalSkillsDir / TestOmpGlobalBridgeDir / TestOmpSessionMetadata
  --- PASS: TestOmpWorkdirSkillsDir / TestOmpInScopeSkillsBases / TestOmpSandboxDirs
  --- PASS: TestOmpSystemContextArgsNil / TestOmpBridgeInjectsExactlyOneSystemBlock

omac start omp starts correctly.

[mwtng]

It would be cool if you could try to make it work via omac start --sandbox <omp-profile> --inner omp using a custom <omp-profile>. Like that we could add only an example profile to the README and it would add zero maintenance overhead for us.

[ludwigku]

It would be cool if you could try to make it work via omac start --sandbox <omp-profile> --inner omp using a custom <omp-profile>. Like that we could add only an example profile to the README and it would add zero maintenance overhead for us.

I tested this approach and it works. I share the config files I used below. With those omp runs successfully in the omac sandbox. The bridge extension included in this PR is still needed to let omp know about omac for proper integration, but can be included in the local omp installation directly.

omp.json
config.yml