Report source commit, public images, unsafe pulls

[jserv]

Four independent changes, one commit each. --version and the self-check report now name the source commit (marked -dirty when src/, build.rs, or the Cargo manifest or lockfile differ from it), so an installed binary older than its checkout is visible instead of silently running old code. An image upload refused for want of a workspace root now gives the real reason, suggests a root that would admit the file, and says a restart is needed. A successful upload reports publicly_readable from one signed-out HEAD of the link (no token, no redirects followed, only to the API's own site), so an agent learns before publishing that an image on a non-guest-readable note will fail for readers. A pull over a file with no usable sync record whose content differs from the note now needs discard_local_changes, and is reported under a new error kind, unverified_local_content, because pushing first cannot help a file with no record.

Behavior change: overwrite_local: true alone no longer replaces such an unrecorded, differing file. A file that already matches the note is still adopted, which is how a broken record is repaired.

Verified with cargo clippy --all-targets --all-features -- -D warnings and make check at every commit (tip: 297 lib tests, 11 stdio tests, live suites ignored), plus cargo build --release and commentflow --check. A second cargo build after a clean one reports the crate Fresh, so the build script does not force rebuilds.

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Summary by cubic

The PR adds four independent changes: --version and the self-check report now name the source commit, image-upload refusals explain the cause and fix, uploads report whether the returned link is publicly readable, and pulls over unrecorded local edits require an explicit discard.

• The reported commit is marked -dirty when src/, build.rs, or the Cargo manifest or lockfile differ from it, even when git settings would hide the edits or name another checkout; a build outside a git checkout omits it, and a failed read inside one is reported rather than passing as a tarball build.
• An upload refused for want of a workspace root now says whether the setting came only from a working-directory .env, suggests a root that would admit the file, and notes that a restart is needed.
• A successful upload now reports publicly_readable from one signed-out HEAD of the link, sent with no token, with no redirects followed, and only to the API's own site; only an image response, or a redirect to presigned storage carrying an access key, signature, and expiry, counts as readable, so an agent learns before publishing that an image on a non-guest-readable note will fail for readers.
• A pull over a file with no usable sync record (none or broken) whose content differs from the note now needs discard_local_changes and fails with the new error kind unverified_local_content, since pushing first cannot help a file with no record.

Behavior change: overwrite_local: true alone no longer replaces such an unrecorded, differing file. A file that already matches the note is still adopted, which is how a broken record is repaired.

^{Written for commit 6481842. Summary will update on new commits.}