[benjaminwood]

Implements the keys half of #58, to the shape described there.

[keys]
left = ["j", "Left"]   # jkl; instead of hjkl
down = ["k", "Down"]
up = ["l", "Up"]
right = [";", "Right"]
line-start = ["^", "Home"]

• [keys] in config.toml: action name to a list of keys. An entry replaces that action's keys; [] unbinds it. Every other action keeps its default.
• Defaults are today's bindings. The existing tests pass unchanged.
• Strict: an unknown action, an unknown key name, or a key bound to two actions that read the same keypress is an error that names it:
  • key "j" is bound to both down and left
  • unknown variant lefty, expected one of up, down, …
  • unknown key "Lft"; expected one character, ctrl+<character>, or one of Up, Down, …
• plannotator-tui config prints every action with its keys.
• Hints follow the bindings: the footer, the toolbar, the Review menu rows, the quit question and the visual/move statuses.
• README: a Keys section with the action table; the Where/Keys table notes that it lists defaults.

Shape

• src/keys.rs: Action (kebab-case names), Key (a character, a named key, or ctrl+<character>), Keymap, and one context per mode (GLOBAL, DOCUMENT, MOTION, TREE, MENU, …) listing the actions it dispatches in the order it tries them. Call sites read as before: match self.keys.action(&key, keys::TREE) { Some(Action::Down) => … }.
• An action keeps its name everywhere it applies, so rebinding down once covers the document, tree, notes, menu and pickers.
• Conflicts are checked within the groups of contexts that read the same keypress (global plus the focused pane, visual plus motions, each picker). A key can still mean different things in different modes: Home is top among blocks and line-start for the cursor.
• The bindings were spread across input.rs, menu.rs, pick.rs and archive_view.rs; all four go through the keymap now. The compose box (text entry) is untouched.

For review

• Matching is today's: a plain binding ignores modifiers (shifted letters arrive with Shift set); ctrl+… needs Ctrl alone.
• ctrl+c is its own action, interrupt, so rebinding quit can't lose it. Each mode closes on exactly the keys it did before (e.g. q still doesn't close the Review menu).
• x on a note (remove, also Delete) and x on a block (clear-block) are separate actions, so Delete stays notes-only as today.
• A config that fails to parse still doesn't stop the TUI; it falls back to defaults, as [ui] theme does, and plannotator-tui config reports the error.
• The picker and menu borders (↑↓ … enter … esc) still name arrows and fixed keys; happy to make those follow too if you'd like.

Testing: cargo test --workspace (326 pass; new keys::tests and an app test where jkl; moves and the footer reads jkl; move), cargo clippy --workspace --all-targets clean, cargo fmt --all --check. Run by hand with the jkl; layout above.

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