session-flow/handoff: adopt purpose argument across the save-point engine (aihero lane 1)

[kyle-sexton]

Context

Decided in aihero-course lane 1 (#2899, decision session 2026-08-17): adopt the upstream purpose argument (mattpocock/skills handoff: "What will the next session be used for?") into our save-point engine. Lanes decide but never implement; this item carries the implementation.

Proposed work

• Extend the argument surface of session-flow:handoff and session-flow:continue-in-background from [file|prompt] [topic] to [file|prompt] [topic] [purpose...]: everything after the topic token is optional natural-language purpose text (no quoting, no new syntax). Old invocations parse identically.
• Purpose tailors emphasis only: the Resumption brief leads with it, Suggested skills are selected for it, Remaining actions are ordered by it. It NEVER drops or reorders the mandatory section set (the structure is the anti-drift contract) and NEVER alters the emitted resume-prompt shape (find-handoff's detection contract; a shape change would be a knowing contract break requiring a coordinated find-handoff change, which this item does not make).
• A purpose that contradicts the Original goal is flagged at write time, not silently obeyed: Original-goal immutability wins.
• Update save-point.md and both producers' argument-hint frontmatter; keep parsing in $ARGUMENTS, not pre-compute (fix(skills): pre-compute Class 2 remainder — genuine shell expansion mixed into pre-compute in 5 plugins (canonical tracker; scope in comments, body list superseded) #1687 constraint).

Acceptance criteria

• Both producers accept the trailing purpose text; existing two-argument invocations unchanged
• Tailoring rules stated in the engine doc: emphasis only, never drops sections, resume-prompt shape untouched, Original goal wins on conflict
• find-handoff detection contract untouched (no change needed there)
• Evals updated where the argument surface is asserted

References

• session-flow/handoff: aihero course lane 1: use-case boundaries, purpose argument, expiry #2899 (lane decision record) and docs/upstream/aihero-course.md lane 1 rows
• plugins/session-flow/reference/save-point.md, plugins/session-flow/skills/handoff/SKILL.md, plugins/session-flow/skills/continue-in-background/SKILL.md
• Upstream: mattpocock/skills skills/productivity/handoff/SKILL.md (argument semantics)

Metadata

Field	Value
Category	unspecified
Area	session-flow
Ecosystem	unspecified