Merge session checkpoint (P) into agent handoff (H) as a buffer-aware command

[tninja]

C-c a P (session checkpoint) and C-c a H (agent handoff) were two keys doing closely related things, and H only loaded a handoff subtree when the cursor sat exactly on its headline. This merges them into one buffer-aware entry and makes handoff loading work from anywhere inside a handoff section.

Dispatcher. New ai-code-agent-handoff-or-checkpoint in ai-code.el: an Org buffer runs ai-code-agent-handoff (forwarding the prefix arg), an AI session buffer runs ai-code-session-checkpoint, and anything else asks which of the two to run via completing-read so neither existing path is lost. Menu H now points at the dispatcher; the P entry is removed and its key left free. Both original commands remain callable via M-x.

Handoff section detection. ai-code-task.el walks the current heading and its ancestors looking for a handoff headline (* Agent Handoff YYYY-MM-DD HH:MM, matched case-insensitively), so point anywhere inside the section — including under a sub-heading — loads that subtree. Point on any heading still loads it, and body text in a non-handoff section still dumps a new handoff.

Verification. 6 new ERT tests (2 for section detection, 4 for the dispatcher), written failing first: test_ai-code-task.el 25/25, test_ai-code.el 50/50. Full suite sits at the same 13 pre-existing failures as main — no new ones. Clean byte-compile and checkdoc on both touched files. README and HISTORY updated.

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Restore the prefix hint in the handoff menu description

When test/test_ai-code.el runs, ai-code-test-menu-other-tools-includes-agent-handoff-entry requires this suffix description to equal "Handoff / checkpoint (C-u: whole task)", but the changed transient definition supplies only "Handoff / checkpoint". That equality always fails, preventing both the targeted test and the full ERT suite from passing; restore the expected description while retaining the useful prefix-argument hint.

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