docs: apply analysis remediation edits

Applied 4 LOW-severity improvements from /speckit.analyze:

1. Clarified PNR format in FR-005 (TEST-{6 chars} not just 6 chars)
2. Added explicit FR-015 coverage note in tasks.md Phase 3
3. Documented infinite inventory design decision in T020
4. Connected documentation tasks to SC-006/SC-007 objectives

All changes are documentation clarifications, no functional impact.
Analysis showed 100% requirement coverage with zero blocking issues.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

.specify/init-options.json

@@ -0,0 +1,9 @@
+{
+  "ai": "copilot",
+  "branch_numbering": "sequential",
+  "here": true,
+  "integration": "copilot",
+  "preset": null,
+  "script": "sh",
+  "speckit_version": "0.5.1.dev0"
+}