feat(prompt): link local images with Markdown

[clabby]

Summary

• teach conversation agents to use Markdown image syntax for local images
• preserve the instruction in restored sessions by including it with each submission and steer
• leave ordinary display text unchanged

Testing

• cargo check -p tact
• just check-fmt
• just clippy
• just test (946 tests)

Stack

1. This PR
2. cl/image-rendering — nonblocking native image rendering
3. chore(release): prepare 0.6.6 #180 — prepare 0.6.6

No matching open issue exists to close.

[codspeed-hq]

Merging this PR will not alter performance

✅ 18 untouched benchmarks
⏩ 31 skipped benchmarks¹

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_{Comparing cl/image-cleanup (23c2078) with main (7f9a588)}

Footnotes

1. 31 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩