fix(tui): robust clipboard handling with debug logging and headless detection

Problem: Ctrl+C in Hermes TUI shows 'copied' but clipboard often empty.
Root causes:
- Native Linux tools (xclip, wl-copy) require DISPLAY/WAYLAND_DISPLAY; in
  headless Docker/SSH they fail or hang.
- OSC 52 fallback requires terminal emulator support; when absent, sequence
  is dropped silently.
- Dashboard OSC 52 → Clipboard API path fails due to missing user gesture;
  errors were silently caught.
- User feedback 'copied selection' was shown unconditionally, regardless of
  success.

Solution implemented:
- Short-circuit Linux native clipboard probing when no display server is
  present (no DISPLAY and no WAYLAND_DISPLAY). Avoids futile attempts and
  timeouts.
- Add HERMES_TUI_DEBUG_CLIPBOARD env var (1/true). When set, TUI logs to
  stderr which clipboard path is used, probe results on Linux, and whether
  OSC 52 was emitted. Greatly improves diagnosability.
- Improve dashboard clipboard error handling: replace empty catch blocks
  with console.warn messages for OSC 52 decode/Write failures and direct
  copy/paste errors. Makes browser permission/user-gesture failures visible
  in DevTools.
- Add comprehensive clipboard troubleshooting documentation to README and
  AGENTS, covering OSC 52 verification, tmux config, Docker/headless
  constraints, env vars, dashboard caveats, and fallback strategies.

Technical details:
-  in ui-tui/packages/hermes-ink/src/ink/termio/osc.ts:
  - Early return on Linux if both DISPLAY and WAYLAND_DISPLAY unset.
  - Refactor probe sequence to async  with 500ms timeout,
    caching result; subsequent copies use cached tool immediately.
  - Emit debug logs when HERMES_TUI_DEBUG_CLIPBOARD=1.
-  in ink.tsx: log when OSC 52 not emitted (native
  or tmux path in use) in debug mode.
- : OSC 52 handler and Ctrl+Shift+C handler now
  log warnings to console on Clipboard API rejection with error message.
- Documentation: new 'Clipboard Troubleshooting' section in README; new
  'Clipboard environment variables and pitfalls' subsection in AGENTS.md
  (Known Pitfalls).

Tests: full ui-tui test suite (292 tests) passes; clipboard and OSC tests
unaffected. No breaking changes.

Files changed:
- ui-tui/packages/hermes-ink/src/ink/termio/osc.ts
- ui-tui/packages/hermes-ink/src/ink/ink.tsx
- web/src/pages/ChatPage.tsx
- README.md
- AGENTS.md
- CHANGELOG.md (new)

ui-tui/packages/hermes-ink/src/ink/ink.tsx

@@ -1309,11 +1309,11 @@ export default class Ink {
     const text = getSelectedText(this.selection, this.frontFrame.screen)
 
     if (text) {
-      // Raw OSC 52, or DCS-passthrough-wrapped OSC 52 inside tmux (tmux
-      // drops it silently unless allow-passthrough is on — no regression).
       void setClipboard(text).then(raw => {
         if (raw) {
           this.options.stdout.write(raw)
+        } else if (process.env.HERMES_TUI_DEBUG_CLIPBOARD) {
+          console.error('[clipboard] [osc52] no sequence emitted — native clipboard or tmux buffer path in use')
         }
       })
     }