Implement GitWorkspaceState classifier and Git probe severity discipline

[Copilot]

Terminal/workspace watchers were probing non-repo directories, .git dirs, detached/bare/corrupt repos, and superseded watcher states without typed classification, producing warning spam for expected negative conditions.

New module: src/agent_term/git_workspace.py

• GitWorkspaceState enum — ten typed states: not_a_repo, repo_root, inside_worktree, inside_git_dir, bare_repo, worktree_no_remote, detached_head, corrupt_repo, permission_denied, operation_superseded
• severity_for_state() — severity-discipline table; expected negative states (not_a_repo, inside_git_dir, bare_repo, worktree_no_remote, detached_head) are info by default; operation_superseded is debug; corrupt_repo/permission_denied are warning; severity escalates to warning only when the caller signals the pending op actually requires the missing capability
• classify_git_workspace(path, *, force=False) — runs git rev-parse sub-commands, detects .git dir probing before any branch commands, catches PermissionError from both os.listdir and git stderr, detects corrupt repos from stderr keywords; results cached by resolved absolute path (force=True bypasses cache)
• GitWorkspaceClassification dataclass — carries state, path, branch, remote_status, severity, action_hint; to_status_line() renders the compact terminal UX format; to_metadata() emits adapter-ready dict

clf = classify_git_workspace("/some/dir")
print(clf.to_status_line())
# Workspace: /some/dir
# Git state: worktree_no_remote
# Remote: missing
# Branch: main
# Severity: info
# Action: Run `git remote add origin <url>` to add a remote.

# Severity escalates only when the op actually needs the missing capability
severity_for_state(GitWorkspaceState.WORKTREE_NO_REMOTE, operation_requires_remote=True)
# → "warning"

Tests: tests/test_git_workspace.py

27 tests covering every state, cache/debounce behaviour, severity-discipline invariants, status-line format, and metadata shape. All git I/O faked via unittest.mock.patch on _run_git.

[mdheller]

Review: good classifier foundation, but not contract-complete yet

This PR has a strong start: typed state enum, severity discipline, caching/debounce, status-line output, and broad tests. CI is also reported successful on the PR head.

However, before this should be marked ready/merged, it needs to align directly with the merged upstream contract from SourceOS-Linux/sourceos-spec PR #101.

Required remediation:

1. Add a GitWorkspaceState receipt/emission shape that matches upstream schemas/GitWorkspaceState.json exactly enough for downstream validation.
2. Add or copy a canonical fixture equivalent to upstream examples/gitworkspacestate.json.
3. Add validation against that schema/fixture, or a documented sync path from sourceos-spec.
4. Align enum spellings with upstream where emitted as receipt data. In particular, upstream uses remoteState = not_applicable, while this module currently exposes not-applicable as a terminal UI constant. UI text can keep hyphenated display text, but emitted receipt metadata should match upstream schema.
5. Ensure emitted receipt data includes the upstream-required fields: sessionRef, capabilityLedgerRef, workspaceRef, state, severity, operation, actionability, pathEvidence, gitFacts, policyDecisionRef, non-empty evidenceRefs, and capturedAt.

Recommendation: keep the classifier as the implementation core, then add a thin adapter such as to_git_workspace_state_receipt(...) that produces the upstream contract payload without forcing terminal UI internals to become schema-shaped everywhere.