Add p.inputField helper; clarify readInput/maxTurns/tools docs

[Copilot]

Daily evaluation identified several undocumented patterns and missing API ergonomics. The ${"input.fieldName"} workaround for referencing non-path input values in prompt templates had no type-safety, no autocomplete, and no documentation.

New API: p.inputField(field)

Returns "input.<field>" for explicit reference to a caller-supplied input field in prompt prose — a documented, type-safe replacement for the opaque string literal:

// Before: opaque, undocumented, looks like a no-op literal
instructions: p`Merge config files listed in ${"input.files"}.`

// After: explicit, discoverable, correct semantic
instructions: p`Merge config files listed in ${p.inputField("files")}.`

Use p.inputField when the field value itself (array, string, number) should appear in prose. Use p.readInput(field) when the field holds a single file path whose contents the model should read.

Documentation fixes

• SKILL.md / prompt-intents.md: p.readInput now explicitly says "single path" (not array); added p.inputField to the helper table and a dedicated "Referencing non-path input values in prose" section with example
• composition.md: concrete maxTurns counting — maxTurns: 3 = 1 initial attempt + 2 repair retries
• agent-api.md: added defineTool vs p.bash decision table covering 6 situations
• README.md: added p.readInput and p.inputField to prompt intents section

• Fixes [rig-tasks] Daily rig evaluation — 2026-07-24 — 5/5 passed #99