feat(probe): add tmux and ssh discovery

[KofTwentyTwo]

Summary

Adds Milestone 2 discovery plumbing for limen: tmux session parsing, deterministic SSH argv construction, and injectable localhost/remote probing. This stays below the UI and exec-handoff boundary.

Scope

• This PR matches a behavior described in docs/DESIGN.md.
• If the PR changes intended behavior, the design doc is updated in this PR (or a preceding one).
• If the PR adds a non-trivial dependency, the rationale is in the summary above.

Changes

• Adds internal/tmuxinfo for local and remote tmux ls -F parsing.
• Adds internal/ssh for local tmux argv, interactive SSH session argv, and remote probe argv.
• Adds internal/probe with injected command execution, per-host timeout handling, no-server semantics, latency capture, and streaming result coordination.
• Adds unit coverage for parser edge cases, SSH argv combinations, local/remote probe outcomes, timeout behavior, and concurrent probe startup.

How to test

go test -race -count=1 ./...
go vet ./...
go build ./...
go test -cover -count=1 ./...
nix flake check

Screenshots / asciinema

Not applicable. This milestone intentionally has no TUI.

Checklist

• Tests added or updated, where appropriate.
• go vet ./... passes.
• No new TODOs that aren't tracked in an issue.
• Commit messages follow Conventional Commits (feat:, fix:, docs: ...).

Closes #2