fix(herdr): populate workspace path and show pane count in detail

[lmilojevicc]

Summary

Under herdr the workspace detail pane showed an empty path and always-unknown activity/created. This derives each workspace's path from the focused pane's cwd, adds a panes (splits) row, and makes the activity/created rows data-driven so they hide when no timestamp exists. tmux behavior is unchanged.

Closes #35

Changes

• Path (was empty): herdrBackend.ListSessions derives each workspace's path from the focused pane's foreground_cwd/cwd via a new best-effort herdr pane list call (mirrors the existing tab-list pattern), falling back to the first pane, then ws.Cwd. workspace list exposes no cwd, so panes are the only source.
• Splits (new): new Item.Panes field, populated from the workspace pane_count that workspace list already returns; the detail pane shows it as a panes row when > 0.
• Timestamps (were always unknown): herdr exposes none, so detailLines now renders activity/created only when the time is non-zero. tmux (which always sets them) is byte-identical; herdr cleanly omits them. Future-proof — if herdr adds timestamps, the rows reappear automatically.
• workspaceInfo parses pane_count; new herdrPanes / herdrWorkspacePaths helpers (single-pass: focused pane cwd wins, else first-seen).

Validation

• mise run verify — fmt:check / lint (0 issues) / vet / build clean; all tests pass
• Tests added/updated:
  • TestHerdrBackendListSessions — path from focused pane foreground_cwd + Panes from pane_count
  • TestHerdrBackendListSessionsFocusedPath — focused pane wins even when not first; foreground_cwd preferred over cwd
  • TestHerdrBackendListSessionsNoFocusedPaneUsesFirstPaneCwd — first-seen fallback, disambiguated from ws.Cwd
  • TestHerdrBackendListSessionsPaneListErrorFallsBackToWorkspaceCwd — best-effort error-swallow contract
  • TestSessionDetailShowsPanesAndTimestampsWhenSet — positive branch guard
  • TestSessionDetailHidesPanesAndTimestampsWhenZero — negative branch guard
• Docs updated (README/AGENTS.md) if needed — none needed (internal display metadata only)

Notes

• The one failing test locally, TestApplyManifestFallbackClaudeBlockedSurvivesIdleRule, is pre-existing and unrelated — it fails because agents_capture_test.go reads the real user manifest cache (a newer upstream claude.toml), and passes in CI which has no cache. Tracked separately in Manifest tests read real cache; newer upstream claude.toml makes local verify red #36.
• Item.Panes is intentionally not exposed in --json output (out of scope for this TUI-focused issue).