Honor --dry-run when staging files

[piyushbag]

Summary

lore stage --dry-run computed staged state but still persisted the staged anchor, so a later real stage reported "No changes staged" and commit could pick up the preview state.

• Guard staged-anchor persistence with globals.dry_run() in file/stage.rs and stage.rs.
• Keep stage end/progress/revision events on dry-run so preview output is unchanged.

Fixes #125

Test plan

• cargo +nightly fmt --all
• cargo clippy -p lore-revision --all-targets -- -D warnings --no-deps
• cargo test -p lore-revision --test stage stage_dry_run_no_persist

Evidence

$ cargo test -p lore-revision --test stage stage_dry_run_no_persist
running 1 test
test tests::stage_dry_run_no_persist ... ok

test result: ok. 1 passed; 0 failed; 0 ignored; 0 measured

The test commits a baseline revision, modifies a tracked file, runs dry-run stage (no staged anchor persisted), then runs a real stage and verifies the staged anchor is written.