feat(tui): reserve / for skill autocomplete, move commands to ctrl+p exclusively

[EngineerProjects]

Summary

Currently / opens the commands palette and also triggers slash commands
like /clear. This PR splits the two concerns cleanly:

• / → skill autocomplete only (injects context into the prompt)
• ctrl+p → all system commands (clear, compact, sessions, model, quit…)

Rationale

Skills inject context into the current message — they are part of the
input composition flow. Commands act on the system — they belong in the
command palette. Mixing them under / forces users to remember which
slash-words are skills and which are commands.

Changes

/ behaviour (new)

• Typing / on an empty input opens a skill autocomplete overlay
• Lists skills from .nexus/skills/ filtered as the user types
• enter injects @skill-name (or the skill content) into the input
• esc closes without inserting

Commands moved to ctrl+p only

• /clear → available in palette as "Clear chat"
• /compact → available in palette as "Compact context" (see feat(engine): manual /compact command and configurable compaction threshold #22)
• Any other slash commands → palette entries

Shortcut for compact (proposal)

• ctrl+b added as a direct shortcut for "Compact context"
• Note: conflicts with tmux default prefix — alternatives: ctrl+k, ctrl+/
• Final key TBD during implementation

Acceptance criteria

• Typing / shows skill autocomplete, not command palette
• Skills filtered in real time as user types after /
• No slash commands remain (/clear, /compact removed as text triggers)
• All former slash commands accessible via ctrl+p
• A direct shortcut added for compact (key confirmed during implementation)
• Footer hints updated
• No regression in existing ctrl+p palette behaviour