docs: long-form strategy worked example in AGENTS.md

[kraftbj]

Extends the upstream-contribution policy in AGENTS.md with a second worked example — the long-form Bluesky strategy (DOTCOM-16810).

The design it documents is still in flight:

• Upstream wordpress-atmosphere#34 (draft) adds the `atmosphere_long_form_composition` filter, the composition methods, the `META_THREAD_RECORDS` constant, and the thread-aware Publisher redesign.
• FOSSE-side `Long_Form_Strategy` projector will ship in a follow-up PR once upstream merges (Task 3 of the SDD plan).

The worked example references both. Both are fair game to describe even pre-merge — the SDD has locked the shape.

Notes

• No `readme.txt` entry in this PR. FOSSE has no `readme.txt` today and nothing in the build / CI references one. The SDD plan mentioned one, but creating it would be a separate decision from this docs tweak.
• Staying as a draft until the upstream + follow-on FOSSE PRs are further along.

Fixes DOTCOM-16890
See DOTCOM-16810 (parent epic)

[Copilot]

Pull request overview

Adds a second “worked example” to the upstream-contribution policy in AGENTS.md, documenting how the upcoming long-form Bluesky strategy is intended to split responsibilities between upstream wordpress-atmosphere and FOSSE.

Changes:

• Document a new worked example for the long-form Bluesky strategy (DOTCOM-16810).
• Describe which planned components belong upstream vs. in FOSSE (incl. the Long_Form_Strategy projector and new upstream filter + composition/publish changes).
• Add a pointer to the long-form strategy SDD for the decision record.