feat(conversation): add :rename command to set custom conversation titles

[tivris]

Summary

Adds the ability to rename conversations with a custom title, so they show up with meaningful names in /conversations instead of just UUIDs.

• Interactive mode: /rename <name> (alias: /rn) renames the current conversation
• CLI mode: forge conversation rename <id> <name> renames any conversation by ID
• Leverages the existing title field in the conversation schema — no migrations needed
• Input is trimmed and validated (empty names are rejected)

Closes #2759

Changes

File	What changed
forge_api/src/api.rs	Added rename_conversation to the API trait
forge_api/src/forge_api.rs	Implemented via modify_conversation
forge_main/src/model.rs	Added Rename(String) variant, /rename + /rn parsing, reserved command registration
forge_main/src/cli.rs	Added Rename subcommand to ConversationCommand
forge_main/src/ui.rs	Added dispatch + handler for both interactive and CLI paths
forge_main/src/built_in_commands.json	Added command entry
forge_repo/src/conversation/conversation_repo.rs	Added tests for title persistence

Test plan

• All 7 new tests pass (cargo test --lib -p forge_main -- rename + cargo test --lib -p forge_repo -- rename)
• All 1000+ existing tests still pass
• cargo clippy clean on all changed crates
• Parsing: /rename my-session, /rn auth-refactor, /rename auth refactor work (multi-word)
• Error cases: /rename with no args returns helpful error
• Whitespace trimming: /rename padded name → padded name
• Title persistence: rename via upsert, rename from None

[CLAassistant]

All committers have signed the CLA.