Fix amp agent prompt extraction to use TUI box pattern

Amp CLI runs in TUI mode with box-drawing UI (│ text │) similar to
Cursor, not shell-style (> prompt). Updated prompt pattern from
`(?m)>\s*(.+)$` to `(?m)│\s*(.+?)\s*│\s*$` to correctly extract
text from amp's box UI.

Changes:
- internal/tmuxedit/config_agent.go: Update amp promptPat to box pattern
- internal/tmuxedit/config_agent_test.go: Update test to use box format
- docs/usage.md: Document detection order and clear methods for all agents
- docs/tmux.md: Clarify input mode handling (Vim vs Emacs/readline)
- config.toml.example: Add detailed agent descriptions and patterns
- prompts/tmux-edit-integration-tests.md: Add test status, mock editor best practices

Integration tests verified:
- Amp detection: amp/sourcegraph keywords
- Prompt extraction: "hello world test" correctly captured
- End-to-end workflow: text modification and sending works
- Multi-line support: all lines delivered correctly
- All unit tests pass (67/67)
- Coverage: 80.9% (meets requirement)
- Cursor and Claude implementations unchanged

Co-authored-by: Cursor <cursoragent@cursor.com>

1 files changed, 60 insertions, 0 deletions

diff --git a/docs/coverage/_internal_textutil.out b/docs/coverage/_internal_textutil.out
new file mode 100644
index 0000000..a03334b
--- /dev/null
+++ b/docs/coverage/_internal_textutil.out
@@ -0,0 +1,60 @@
+mode: count
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