docs(context): add RULESET.TESTS.boundary-coverage + LEARNING.prompt-budget.boundary-gap

Adds four predicates derived from PR #3708 post-mortem:
- RULESET.TESTS.boundary-coverage — must test {limit-1, limit, limit+1} and near-reserve-distance inputs
- RULESET.TESTS.boundary-coverage.fixtures — required fixtures for budget/limit/quota/threshold code
- RULESET.TESTS.boundary-coverage.anti-pattern — names the trivially-large + trivially-small pairing as the failure mode
- LEARNING.prompt-budget.boundary-gap — full post-mortem of the UNNEEDED_TRIM + FALSE_HARDFAIL regressions

CONTEXT.md

@@ -123,6 +123,10 @@ Five-axis story decomposition discipline (**S**pike, **P**aths, **I**nterfaces,
 `RULESET.TESTS.guard-toplevel-readFileSync=module-level const src = readFileSync(...) throws before any test() registers — wrap in try/catch in test() or use lazy load`
 `RULESET.TESTS.coderabbit-fix-prefer=behavioral tests (call exported fn, capture JSON, assert typed fields) over source-grep`
 `RULESET.TESTS.diagnostics=after JSON.parse, assert output shape (Array.isArray(output.phases)) with raw-output-prefix diagnostics before .map() — prevents opaque TypeErrors when CLI output shape changes`
+`RULESET.TESTS.boundary-coverage=tests MUST exercise inputs at and near the threshold/limit, not only trivial-fit and trivial-overflow; pick inputs where N ∈ {limit-1, limit, limit+1} and where pre-trim/pre-check accumulators ≈ effective limit; "very small" and "very large" inputs alone do not constitute edge-case coverage and routinely miss off-by-one + reservation-accounting bugs`
+`RULESET.TESTS.boundary-coverage.fixtures=for any code with budget/limit/quota/threshold parameter, test suite MUST include: (a) input where SUT estimate == limit exactly, (b) input where estimate == limit - 1, (c) input where estimate == limit + 1, (d) input where any internal reserve/safety constant pushes baseline within reserve-distance of limit (catches early-pressure firing)`
+`RULESET.TESTS.boundary-coverage.anti-pattern=test suites that pair budget:1_000_000 (trivially fits) with budget:1 (trivially overflows) and skip the boundary region; failure mode that shipped PR #3708 UNNEEDED_TRIM + FALSE_HARDFAIL regressions (commit 2df566ed, fixed bde1ae8f)`
+`LEARNING.prompt-budget.boundary-gap=PR #3708 commit 2df566ed reserved NOTE_RESERVE_TOKENS in pressure-threshold AND in minSet pre-check; both buggy paths only fire when baseTokens ∈ (effectiveBudget - NOTE_RESERVE_TOKENS, effectiveBudget]; original test suite used budgets far from that band so neither path was exercised; fix bde1ae8f confines NOTE_RESERVE accounting to post-trim assembly path only; future budget/limit code MUST add boundary fixtures per RULESET.TESTS.boundary-coverage.fixtures`
 
 `RULESET.WORKFLOW_MARKDOWN.FENCES=preserve opening language fence when editing shell snippets in workflow markdown; malformed fence creates fresh CR threads (MD040)`
 `RULESET.WORKFLOW_SIZE_BUDGET=workflow-size-budget can fail otherwise-valid review fixes; XL workflows <=1800 lines or trim prose before final checks`