chore: default persistent_facts to an empty array (#2750)

* chore: default persistent_facts to an empty array

Every customize.toml shipped with a skill seeded persistent_facts with
file:{project-root}/**/project-context.md. That made project-context an
opt-out default rather than an opt-in customization. Ship the arrays empty
so nothing is loaded unless the user adds it.

* docs: correct the persistent_facts default in comments and docs

Comments and docs still described project-context.md as loading by default.
They were wrong twice over: the array now ships empty, and bmad-project-context
no longer produces a project-context.md at all — it writes a verified block into
AGENTS.md and treats project-context.md as a legacy artifact.

Replace those claims with the actual model: repo-wide context belongs in
AGENTS.md, which every skill already sees; persistent_facts carries context only
one skill needs, loaded on demand instead of as constant memory. Each site shows
the file: entry users can add to opt back in.

docs/reference/build-auto.md

@@ -78,8 +78,7 @@ On activation, the workflow resolves:
 
 - `_bmad/config.toml`, `_bmad/config.user.toml`, and optional team/user overrides under `_bmad/custom/`
 - Any configured workflow customizations from `customize.toml`, team overrides, and user overrides
-- Persistent facts listed in workflow config
-- `project-context.md` files, if present
+- Persistent facts listed in workflow config — empty unless you opt in, so nothing is loaded here by default
 
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