feat(tui): add secure session copy and export

[ZhiXiao-Lin]

Summary

• add /copy for the latest assistant source Markdown and /copy transcript for the semantic session
• add /export [path] for atomic, private, no-clobber Markdown snapshots inside the workspace
• preserve visible messages, tool evidence, and delegated results while excluding private reasoning and transient terminal chrome
• report verified native clipboard delivery separately from bounded OSC 52 requests

Safety

• explicit export paths must be workspace-relative and have an existing in-workspace parent
• parent symlinks that escape the workspace are rejected
• writes use a temporary file, persist_noclobber, file sync, directory sync, and Unix mode 0600
• existing targets are never overwritten
• OSC 52 payloads are capped at 64,000 UTF-8 bytes without splitting a character

Validation

• cargo fmt --all --check
• cargo check --all-targets --locked --offline
• cargo clippy --all-targets --locked --offline -- -D warnings
• cargo test --all-targets --locked --offline (1,468 main TUI tests passed, 4 ignored; all integration targets passed)

Recovered narrowly from snapshot ded6c8ff54c86637d5050900fa7a8a01ca9df0bb; concurrent terminal, rewind, task, permission, and Use work is intentionally excluded.

[ZhiXiao-Lin]

Integrated through #40 at merge commit c252583e413f70a4cb265012d776f9d050b94de5. The consolidated v0.9.7 branch contains this PR's latest head 0763f93847b1c8a586cdbfe6f2edaada09711aef and passed Linux, macOS, and Windows CI.