tui: fast-copy keybinds for selected node hierarchy (y/Y)

[plattnum]

Summary

Adds two clipboard shortcuts to plnk-tui for handing the currently selected node off to an AI agent (or any paste target) without retyping IDs.

Key	Output
y	Compact JSON of the selection's ID hierarchy (project / board / list / card, each with id + name)
Y	Paste-ready plnk <resource> snapshot --output json command, preceded by a breadcrumb comment

Both keys write to the system clipboard via the OSC 52 escape sequence. No native clipboard dependency — works locally, inside tmux (with set -g set-clipboard on), and over SSH.

Handles all six TreeKey variants: project, board, list, card, grouped card, and label group (which resolves to its underlying list).

Security

Project / board / list / card names are user-editable on the Planka server. A name containing \n would otherwise break out of the leading # comment line in the Y output and put attacker-controlled text on its own line — executable on shell paste. The breadcrumb embedded in the shell command form has all control characters (C0, DEL, C1) stripped via char::is_control(), replaced with a single space. The JSON form is unaffected because serde_json escapes control characters by definition.

Test plan

• cargo test -p plnk-tui — 13 new unit tests pass
  • RFC 4648 base64 vectors + edge bytes
  • All six TreeKey variants resolve correctly
  • JSON escaping for ", \\, \n
  • Newline-injection attack on shell form blocked (exact 2-line output)
  • \r, \x1b (ANSI escape), \x00 stripped from breadcrumb
• cargo test — all 144+ workspace tests pass
• cargo clippy --all-targets -- -D warnings — clean
• cargo fmt --check — clean
• Manual: launch plnk-tui, select a card, press y, paste — confirm JSON arrives on clipboard
• Manual: same flow with Y — confirm breadcrumb + snapshot command arrives
• Manual: verify behavior over SSH and inside tmux

Docs

• docs/tui/fast-copy.md — new feature page (formats, OSC 52 mechanism, terminal compatibility table, security model)
• docs/tui/keybindings.md — y / Y rows under Global
• docs/tui/overview.md — feature bullet + cross-link
• README.md — one-liner + docs link