Add generated theme-aware capability-matrix figure

[jothimani-rajendran]

What this changes

Adds docs/figures/ (the open-coder-ai launch programme's shared figure tooling, block 2):
a capability-matrix figure and a social card generated from src/agentseam/matrix.py, plus
the shared family diagram carried byte-identical by every sibling repository. Embeds all
three in README.md and adds a CI job that regenerates every docs/figures/make_*.py and
fails the build on any diff.

• docs/figures/palette.py, make_family.py, family-{light,dark}.svg — copied verbatim
  from the shared programme source; sha256sum verified identical at copy time and after
  every local regeneration.
• docs/figures/make_matrix.py — renders matrix-{light,dark}.svg: 16 agents × 12 events,
  read from agentseam.matrix at render time. Every cell is grade + a two-letter glyph
  (colour is never the only carrier); "none" (no hook surface) renders in the shared
  NEUTRAL grey, never the pale end of the graded ramp; "enforced" is asserted against the
  data and, since no cell reaches it, is named explicitly rather than silently omitted.
• docs/figures/make_social.py — renders social-card.{svg,png} (1280×640, light only):
  the repo name, one line, and one true number pulled from the same tally the matrix figure
  computes, so the two can never disagree.
• .github/workflows/ci.yml — new figures job, pinned to the SHAs already used elsewhere
  in this workflow.

Claim check

Not applicable — no MATRIX row or capability claim changes. The figure is read-only over
existing, already-verified data; nothing here widens what any adapter is claimed to do.

Checks

• pytest -q passes — 1598 passed, 4 skipped (unchanged from main)
• ruff check . and ruff format --check . pass
• Runtime path is still stdlib-only — docs/figures/ is design tooling, not
  src/agentseam; its generators import only palette (this repo) plus
  agentseam itself, and cairosvg only inside a guarded, optional PNG step for the
  social card that the CI figures job never needs (it only regenerates and diffs SVGs)
• Commits are signed off (git commit -s)

Notes for the reviewer

• Verified facts re-derived from the code, not typed by hand: len(matrix.MATRIX) == 16;
  the 12 events are exactly pre_tool, post_tool, stop, tool_failure, prompt_submit, session_start, session_end, pre_compact, subagent_start, subagent_stop, instructions_loaded, file_changed (there is no file_write event); at pre_tool, 11
  agents are best-effort, Cursor alone is enforceable, and aider/copilot/replit/
  zed are none — no agent anywhere in the 192-cell matrix is graded enforced.
  enforced=0, enforceable=2, best-effort=33, detect=56, none=101.
  docs/figures/make_matrix.py cell_grades() computes this each run; make_social.py
  imports that same function rather than recomputing it.
• The pyproject.toml ruff format exclude that an earlier revision added for the two
  shared files has been dropped: the shared source was corrected upstream so both files now
  pass ruff check and ruff format --check untouched, with byte-identical SVG output.

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