docs: rename AGENT.md to AGENTS.md, fix stale/inaccurate content

[sensei-hacker]

Summary

• Renames AGENT.md to AGENTS.md (the emerging convention; no internal references to the old filename found).
• Fixes several inaccuracies found while verifying the file against current source:
  • The branching section claimed master was actively developed on and merged forward — corrected to describe it as a maintainer-maintained lagging mirror that contributors should never target or branch from (matches the corrected description landing in Development.md via docs: fix master-branch merge-flow error in Development.md, add git workflow guidance #11742).
  • Hardcoded version string ("9.0.1") replaced with a pointer to CMakeLists.txt's project(INAV VERSION ...), so it can't go stale again.
  • Removed an unsubstantiated "Follow MISRA C rules" claim — grepped the whole repo, there's no MISRA enforcement anywhere.
  • Fixed the gyroConfig_t parameter-group example (real fields are gyro_main_lpf_hz/kalman_q, not gyro_lpf_hz/gyro_kalman_q) and the DMA example (dmaGetByRef(), not a nonexistent dmaGetIdentifier()).
  • Added a "Development Documentation" section pointing at what's actually in docs/development/ today.

Test plan

• Every claim above independently verified against current source (grep for internal AGENT.md references, CMakeLists.txt version, gyroConfig_t struct, dma.h function signatures, MISRA grep across the repo, recent merged PRs targeting master)
• Docs-only change, no functional code touched

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