docs: make syntax.mdx the single source of truth for macros (S2: reference hub)

[austenstone]

Session 2 — Reference hub & de-duplication

Part of the 4-session docs overhaul. Fixes maintainer complaint #2: the macro reference was duplicated and inconsistent across pages. This makes /docs/syntax the single source of truth for the keyword dictionary and stops hand-maintaining macro lists elsewhere.

Changes

docs/content/docs/syntax.mdx (the SSOT)

• Tightened intro to canonical positioning: superset of workflow YAML + a handful of compile-time macros, actio build expands them, zero runtime / zero lock-in, macro-free file already works. States this page is the canonical keyword dictionary and /docs/macros/* are the deep dives.
• Replaced the single quick-reference table with scannable tables grouped by scope (Workflow / Job / Step / Lifecycle), folding sub-keywords under their parent rows. fragments row flagged deprecated.
• ⚠️ No inventory changes: all 35 {/* macro:X */} markers and 11 <TypeTable>s untouched (schema-locked, machine-enforced).

docs/content/docs/glossary.mdx

• Deleted the stale 9-entry ## Macro keywords section and trimmed the inline keyword list in the Macro term → both now point to /docs/syntax + /docs/macros.
• Added Executor and Hoist concept definitions; kept all genuine terms.
• Dropped "fragment" from the frontmatter description.

AGENTS.md

• Replaced the stale 11-row macro table with the reconciled ~19-keyword taxonomy grouped by job (Reuse / Config presets / Matrix / Reliability / Wiring / Conditionals / Security / Artifacts) + a pointer to the syntax reference for advanced keys (let, coercion, finally, hoist overrides). Example block (already _anchors/params/retry-led) kept.

Verification

• docs-completeness 3/3 ✓
• syntax-reference 15/15 ✓ (35 markers + 11 TypeTables intact)
• macros 66/66 ✓
• npm run docs:build ✓ (all 154 pages prerendered)

Coordination note: meta.json / macros/meta.json intentionally untouched — the _anchors.mdx/templates.mdx slug-add is owned by Session 3.