feat(ui): name the operation in completing-read prompts

A picker prompt is the last thing shown before a command commits, so a bare noun leaves a mis-keyed command ambiguous. Hitting C-f8 (project agenda) instead of C-f9 (AI-vterm picker) gave the same "Project:" prompt with no signal which one was about to run.

Reworded 17 prompts across 8 modules so each names the operation rather than just the thing being chosen: "Project:" becomes "Show agenda for project:", "F6:" becomes "Run tests:", the dwim-shell sub-prompts gain their context (checksum algorithm, PDF compression quality, text-to-speech voice, run dwim-shell command), the two contact pickers split into "Find contact:" and "Insert contact email:", and the dirvish ediff, org finalize, and custom-comments length/box-style prompts get the same treatment.

I audited all ~124 completing-read / read-* call sites; the rest already named their operation and were left alone. These are prompt-string changes only, no logic touched.

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