Publish undocumented shortcuts

[peters]

Summary

• publish the missing documented shortcuts in the README, including the existing sessions shortcut and the built-in copy, paste, search, and SSH reconnect flows
• add the missing editable Search Terminals binding to the Settings UI and add a read-only built-in section for contextual shortcuts
• centralize SSH reconnect conflict detection so the panel behavior and Settings copy stay in sync

Why

Issue #164 called out several shortcuts that already existed in the app but were not published anywhere users would discover them. The audit also found the sessions shortcut was missing from the README table and the settings editor had no row for the configurable search binding.

Validation

• cargo fmt --all -- --check
• ./scripts/check-maintainability.sh
• cargo test --workspace
• cargo clippy --all-targets --all-features -- -D warnings
• cargo clippy --workspace --lib --bins --examples -- -D warnings -D clippy::unwrap_used -D clippy::expect_used
• cargo clippy --workspace --all-targets --all-features -- -D warnings -W clippy::pedantic
• launched target/debug/horizon with an isolated HOME, opened Settings -> Shortcuts, and captured live screenshots before and after resizing the window during the smoke pass

Closes #164.

[Copilot]

Pull request overview

This PR makes existing but previously undiscoverable keyboard shortcuts visible to users by documenting them in the README, exposing the missing configurable “Search Terminals” binding in the Settings UI, and centralizing SSH reconnect shortcut conflict detection so UI messaging and behavior stay consistent.

Changes:

• Add a centralized shortcut inventory module for global shortcut enumeration, SSH reconnect conflict detection, and platform-specific copy/paste labels (with tests).
• Update the Shortcuts Settings tab to render configurable shortcuts via an enum-driven list, add “Search Terminals”, and display a read-only “Built-In Shortcuts” section with contextual availability notes.
• Publish previously undocumented shortcuts in the README (sessions picker, terminal search focus, copy/paste, SSH reconnect) with clarified platform notes.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
crates/horizon-ui/src/app/shortcut_inventory.rs	New centralized helpers for shortcut inventory + conflict detection + copy/paste labels, with unit tests.
crates/horizon-ui/src/app/settings/shortcuts.rs	Adds editable “Search Terminals” row, refactors editable shortcut rendering, and adds read-only built-in shortcuts section with SSH reconnect availability messaging.
crates/horizon-ui/src/app/panels.rs	Replaces duplicated SSH reconnect conflict logic with the centralized helper; removes now-redundant tests.
crates/horizon-ui/src/app/mod.rs	Registers the new shortcut_inventory module.
README.md	Documents previously missing shortcuts and expands platform-specific notes for copy/paste and contextual SSH reconnect behavior.

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