[ghcp-handoff] T1: Expand from-plan heading aliases or add marker comment

[bminier]

Problem

`bin/ghcp-extract-plan.ts` only recognizes `## The GHCP prompt` as the handoff heading. Real-world plans vary widely.

Approach

Accept any of:

• `## The GHCP prompt` (existing)
• `## Copilot handoff`
• `## Handoff prompt`
• `## Delegation prompt`
• OR: a machine-readable marker `` ... `` that wins over heading matching

Marker-comment form is more robust for plans emitted by automation (see G6).

Review reference

Adversarial review T1

[bminier]

Fixed on branch ghcp-t1-heading-aliases (stacked on ghcp-t2-branch-uniqueness).

Two paths now, tried in order:

1. HTML-comment markers — <!-- ghcp:prompt-start --> ... <!-- ghcp:prompt-end -->. Unambiguous; durable across heading-wording drift; can be embedded anywhere in a plan. This is the form /autoplan and plan reviews should emit (addresses most of issue [ghcp-handoff] G6: Integrate from-plan mode with plan-generating skills (or delete it) #6 too).
2. Heading aliases for hand-written plans — ## The GHCP prompt, ## GHCP prompt, ## GHCP handoff, ## Copilot handoff, ## Handoff prompt, ## Delegation prompt, ## The handoff.

Target-branch extraction also broadened to recognize Target branch: \ and Branch: \ in addition to the original against branch \.

ExtractResult grows source + matchedHeading so callers can tell which path fired. ExtractError grows markerPair so the "heading missing" diagnostic can tell users both options.

Will close when merged to main.