docs: add herdr domain glossary (CONTEXT.md)

[brchue-ux]

Summary

Adds a CONTEXT.md domain glossary at the repo root, per the domain-modeling skill's
CONTEXT-FORMAT.md structure (one-line term definitions with _Avoid_ synonyms), and a pointer
to it from AGENTS.md.

Docs-only change: no runtime behavior touched.

Scope and method

Definitions were grounded in the actual source (src/workspace.rs, src/pane/state.rs,
src/pane.rs, src/detect/mod.rs, src/detect/manifest.rs, src/config/sidebar.rs,
src/api/mod.rs, src/server/socket_paths.rs, src/session.rs) and cross-checked against
docs/next/website/src/content/docs/concepts.mdx where user-facing docs already define a term.

Terms covered: session, workspace, tab, pane (PaneState vs PaneRuntime), space, sidebar, agent
detection, manifest, gate, agent state, done (the derived unseen-idle label), presentation, native
agent session, server socket vs client socket, event.

Notes for review — please correct freely

This is a first draft, not a final spec. A few things worth double-checking against intent rather
than just code:

• "Space" is genuinely overloaded in the codebase — it names both a workspace's git/worktree
  identity (GitSpaceMetadata, WorktreeSpaceMembership) and the sidebar's per-workspace summary
  section/config ([ui.sidebar.spaces]). I defined it as "the git/worktree identity a workspace is
  derived from" and noted the sidebar section is named after that concept — but if the intended
  canonical meaning is different, this entry needs correcting.
• Agent state has a 4-vs-5 value mismatch between code and docs. The AgentState enum has 4
  variants (Idle, Working, Blocked, Unknown); user-facing docs describe 5 including Done.
  Done is not a backend state — it's a UI label derived from Idle + PaneState.seen == false
  (src/ui/status.rs). I split this into two glossary entries ("Agent state" and "Done") rather
  than presenting docs' 5-state table as one flat enum — flagging in case that framing is wrong.
• "Presentation" is confirmed live, not deprecated — it's agent-reported display metadata
  (EffectivePresentation: title, display name, per-state labels) layered on top of AgentState.

Test plan

• Docs-only change; no build/test impact.
• Captain review of term accuracy and any missing/wrong entries.

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