The PRRTE AGENTS.md and its PMIx counterpart share the same authorship
and much of the same guidance, but the PMIx file had accumulated several
general, project-agnostic rules that PRRTE's copy was missing.  Carrying
them over keeps the two orientation guides consistent so an agent moving
between the code bases meets the same expectations, and it fills real
gaps in the PRRTE guidance around style, hygiene, and process.

Added: the conditional-spacing style rule; a directive to update
.gitignore for any build product a change introduces; the "never bend a
test to accommodate a bug" testing rule; a shared-repository/worktree
section warning against repo-wide git commands; the preference for
C++-style comments; commit-message guidance (body line wrapping, no AI
tooling attribution, incidental fixes as standalone commits); and a
maintainer-mode caution for build-system edits.

While porting the build-system guidance, correct an over-broad claim in
the existing text: because PRRTE builds in maintainer mode, editing a
Makefile.am does not require the full autogen.pl plus configure cycle —
a plain make regenerates the affected Makefiles.  Also repair a garbled
duplicate line in the copyright-header rule.

Signed-off-by: Ralph Castain <rhc@pmix.org>