fix(copilot): Free plan usage, tall answers in agent read, page-key fallback, review nits

Review of 8659ba25..be987fbb:

- Usage: a Free account answers premium_interactions {entitlement: 0, remaining: 0,
  percent_remaining: 0, unlimited: false, has_quota: false}, which read as 100% used. The parser now
  returns null when has_quota is false or the entitlement is not positive, with that payload as a
  fixture. Free meters chat (200) and completions (2000) too, so the comment no longer says they
  are unlimited on every plan.
- agent read: rows were skipped until a `❯ <prompt>` row, so an answer taller than the pane
  (prompt scrolled off the top) read as "". With no prompt row AND no banner / trust dialog on
  screen, the rows above the composer are now the tail of the last answer. Text typed in the
  composer (a prompt-shaped row between the two rules) is not a prompt row.
- Page keys: measured on 1.0.95 in tmux that Copilot pages on PageUp/PageDown (three PageUp moved
  lines 109-140 to 010-042, PageDown returned) as well as on SGR wheel reports, so copilot joins
  claude and opencode in _localScrollbackIsHollow(); the stale-tracking-flag fallback the comment
  and docs describe is now true, for the main pane and a TerminalTile (same function), with tests.
- X-GitHub-Api-Version is 2022-11-28, a version GitHub supports (verified against the live
  endpoint); readPlanUsageTelemetryEnabled() is checked before isCopilotAvailable() so a switched-off
  chip skips the resolver probe too.
- Palette label for Copilot in panels-ui.js; stale comments in types/session.ts and stock.ts.
- Docs: Past Sessions continues with --continue (resumeSessionId is for API callers), nothing sets
  continueSession on a respawn, and only a tab name the user set is passed as --name; the usage
  wording for Free; the wheel and mouse-strip paragraphs in CLAUDE.md and
  architecture-invariants; the changeset describes the whole feature.

src/types/session.ts

@@ -546,7 +546,10 @@ export interface CopilotConfig {
   allowAll?: boolean;
   /** Resume the most recent session (--continue). Skipped when resumeSessionId is set. */
   continueSession?: boolean;
-  /** Resume a specific session by ID (--resume=<id>). Ids only, never names. */
+  /**
+   * Resume a specific session (--resume <value>): an id, id prefix or session name, as ONE plain word
+   * starting with a letter or digit (a leading `-` would make the value its own flag).
+   */
   resumeSessionId?: string;
 }