fix(memory): enforce readonly mode across writes

[Grivn]

Summary

• make --readonly reject every CLI path that can mutate memory or store selection
• open SQLite through a correctly encoded mode=ro&immutable=1 file URI
• enforce the boundary in the store layer as well as the CLI layer
• document the immutable snapshot semantics in English and Chinese

The write gates cover remember, forget, link, non-dry-run import, mutating embed modes, gc --keep, and store create/set/remove. Read-only paths do not create WAL/SHM sidecars, including when the database path contains spaces.

Verification

• reproduced the bug before the change: --readonly remember exited 0 and increased the row count
• after the change, the same command exits non-zero and the row count is unchanged
• focused CLI/store tests cover write rejection, readable snapshots, path encoding, and sidecar absence
• git diff --check
• make test

Closes #102.