feat: show standard OMP sessions in view-only mode

[wolfiesch]

Summary

• fall back to standard OMP session files when the T4 control bridge is unavailable
• expose only read capability and continue following newly saved transcript entries
• label these sessions OMP · view only with explicit limitations in the transcript and controls
• support default, named-profile, and explicit session roots

Safety boundary

Compatibility mode never starts an OMP worker and rejects prompt, control, and session-management commands at the host even if the UI is bypassed. It does not claim running, idle, or ownership status that standard OMP cannot provide.

Verification

• pnpm check
• pnpm test
• pnpm build:web
• pnpm build:host
• Unix-socket integration test covers discovery, attach, appended saved output, restricted capability negotiation, write rejection, and zero RPC worker spawns
• server-rendered UI contract covers the view-only badge/banner copy and absence of writable controls

Stacked on #95 because that PR introduces the standalone t4-host fallback point.

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[riverpilot]

Not merge-ready:

• The branch is 131 commits behind main, conflicts, and must be rebased against the current official-OMP authority work.
• The fallback constructs FileSessionDiscovery from the configured root without rejecting symlinked JSONL entries or proving each canonical file remains beneath the canonical root. A *.jsonl symlink can expose a transcript outside --session-root through list/load/search. Reject symlinked entries and enforce canonical-root containment, with a regression test.

Current main's opt-in official-OMP authority does not make this exact automatic fallback redundant, so close if that product behavior is no longer desired; otherwise repair and rebase.

[riverpilot]

Implemented the requested symlink rejection and canonical session-root containment directly on current main in #134; host-service typecheck and all 330 tests pass. The original compatibility branch is superseded by current official authority mode and remains conflicted, so please close #98 rather than merging it.

[iarbpairs]

Closing without merge. Current main's official OMP authority supersedes this stale compatibility branch, and the required transcript symlink/canonical-root hardening has landed independently in #134.