docs: update roadmap

[atticusrussell]

Description

Reorganises the roadmap around parallel tracks and brings ARCHITECTURE.md into line with it. Adds CONTRIBUTING.md for working conventions.

Milestones and issues on GitHub have been restructured to match — that half of the change is not visible in this diff.

Roadmap changes

• Track IDs replace M01–M19. Hardware now runs parallel to navigation, so a single linear number asserted an ordering that no longer held. Tracks: NAV, HW, DET, VO, ARM, BT, DEMO.
• Real nav before sim nav. The Gazebo plugins have never been checked against the real robot, so tuning against them first is circular. Sim-nav-as-a-phase is dissolved; a new NAV-3 establishes simulator fidelity against bags recorded during real nav.
• Bench before CAD. Jetson and arm bringup are desk work with no chassis dependency, and doing them first surfaces cable routing and connector reality before plate geometry is committed.
• Behavior tree extracted to its own track. It was previously a second exit criterion on four unrelated capability milestones. A capability milestone should close on whether the capability works.
• DET-0 added. The behavior tree consumes /pickleball/poses and cannot tell which detector produced them, so the existing colour-blob tracker can drive it once it conforms to the contract. Autonomy stops being gated on model training.
• Theory attached to the milestone it unblocks rather than pooled into separate reading milestones.
• Pose-graph SLAM reading added — the backlog covered Gaussian and particle filters but nothing on the algorithm family slam_toolbox actually implements.

Architecture changes

• Milestone references removed; sequencing belongs to the roadmap. Sections describe subsystems rather than phase deltas.
• Open question 6 closed: ball_tracker retained permanently, with a third role as interface stand-in.
• Stale archive/ package rows dropped; ballbot_viz added.
• Decision-log entries added for each of the above. Pre-existing entries keep their original milestone IDs, being a historical record.

GitHub restructuring (not in this diff)

• 12 milestones renamed, 9 created, 5 theory milestones deleted after being emptied
• 21 new issues, including the pose-graph SLAM reading and the DET-0 contract work
• 25 theory issues relabelled and moved into the milestones they unblock
• Dependency graph rewired: bench work no longer blocked on the assembled robot, and BT v1 no longer waits on detector precision
• Every open issue is now either in a track milestone or labelled side-quest

Type of change

• Documentation Update
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update