feat(core): add reset-only /clear command

[xukp20]

Summary

Add a reset-only /clear command for the current session.

This implements the original request from #354: clear the current session context in place without creating a new session entry like /new.

What changed

• add /clear as a first-class command
• keep the current logical session and active session slot unchanged
• clear local history for the current session
• detach the current backend agent session binding
• clean up the current interactive state so the next message starts a fresh backend session
• add regression tests for:
  • clearing the current session in place
  • preserving the current session identity/name
  • workspace-scoped /clear
  • invalid /clear usage
• update i18n strings and usage docs

Why

Today, the only way to start fresh is /new, which creates a brand new session.

That makes /list grow quickly even in cases where the user does not want a new logical session, and only wants to clear the current conversation context while keeping the same session identity.

This PR addresses the original feature request in #354.

Maintainer context on #354:

• the feature was already acknowledged and marked as planned with P2 priority

Scope decision

While implementing /clear, I also evaluated whether cc-connect should support a native clear path in the underlying CLI session.

That native path is intentionally not included in this PR.

Reasons:

1. Codex native slash commands are not reliable enough in exec/resume --json mode
   Related investigation: [Bug] Codex native slash commands are not reliable in exec/resume mode (/compact and /clear) #378

   In particular:

   • /clear is not behaviorally reliable in direct Codex CLI testing
   • /compact also does not appear stable enough to treat as a guaranteed native capability in the current integration mode

2. Claude Code native /clear should be validated separately before exposing it in cc-connect
   Claude Code does support native slash commands in general, but /clear behavior in the current cc-connect integration path still needs separate validation, including Claude-side session/control semantics.

Because of that, this PR keeps the implementation conservative:

• /clear only resets the session at the cc-connect layer
• it reuses the same cc-connect logical session
• it achieves a “fresh start” by dropping the old backend session binding and starting a new backend session on the next user message

Validation

• go build ./...
• go test ./... -v -race
• go test ./... -coverprofile=coverage.out -covermode=atomic
• $(go env GOPATH)/bin/staticcheck ./...
• $(go env GOPATH)/bin/actionlint -color

Related issues

• Fixes [Feature] 支持 /clear 命令原地清空当前 session 上下文 #354
• Related investigation for Codex slash-command reliability: [Bug] Codex native slash commands are not reliable in exec/resume mode (/compact and /clear) #378

[chenhg5]

LGTM. Useful feature for session management.

Review summary:

• ✅ /clear resets session context without creating new session entry
• ✅ Preserves session identity/name
• ✅ Conservative implementation (cc-connect layer only)
• ✅ Tests added
• ✅ i18n updated
• ✅ Fixes #354

Good addition to session management options.