quality(workspace): dedupe workspace_store_image and workspace_pick_and_store_image (R1.4)

[juacker]

What

Extract the duplicated preamble (resolve descriptor + refuse non-agent workspace) and the duplicated ContentPart::Image literal trailer shared by workspace_store_image (paste path) and workspace_pick_and_store_image (file picker path) into two named helpers:

• resolve_agent_workspace_root(state, workspace_id) -> Result<PathBuf, String>
• store_workspace_image(root, bytes, media_type, filename) -> Result<ContentPart, String>

Why

Both commands had the same ~6-line preamble and the same 7-line ContentPart::Image literal (width: None, height: None). Two paths defining the same shape is exactly how they drift — one gets an id, the other doesn't, etc. F4 in docs/arch-review-r1-workspace-rs.md.

Diff at a glance

src-tauri/src/commands/workspace.rs | 88 +++++++++++++++++++++----------------
1 file changed, 50 insertions(+), 38 deletions(-)

The commands shrink; the helpers carry the doc comments explaining why each step exists (one-liner comments at the call sites).

Local evidence

• cargo fmt -- --check: clean
• cargo clippy --lib -- -D warnings: clean
• cargo test --lib: 932 passed, 0 failed (was 932 on main too — no test churn, pure refactor)

Source

Roadmap item R1.4 (file finding F4 of docs/arch-review-r1-workspace-rs.md).

Important note about base ref

This PR targets branch clai/quality/pre-r1-4-main-snapshot (a temporary snapshot of main at 74261a0, the previous tip before this branch was pushed), not the current tip of main (d680a96). The current main contains an extra revert commit (d680a96) that is byte-identical to 74261a0 for the only file this branch touches; it's a leftover artifact of a botched git push -u earlier in this session. See .clai/memory/journal/2026-08-05.md for the full timeline.

To merge cleanly:

• This PR's diff against 74261a0 is the real R1.4 change.
• Before merging, retarget the PR base to main (or merge clai/quality/pre-r1-4-main-snapshot forward to main first). Merging into the snapshot branch leaves main unchanged.

Merge

🛑 Do not merge yet. Per the hard rule added 2026-08-04 ("never merge a PR — this is done by me"), this PR waits for CI green and then human merge.

[juacker]

Superseded: main has moved on (#152, #150) and this PR's head commit 0aa9a16 is now an ancestor of main (it was pushed there by accident, then reverted in d680a96), so it can never be retargeted to main. Re-landing R1.4 as a fresh commit on top of current main in a new PR. See journal entry 2026-08-05 for the full incident timeline.