P3: rename accounts, one set of period defaults, tax categories warn or refuse (defects 15, 16, 8)

books.rename_account; account names trimmed, bounded, refused when a
sibling's or TIN-shaped; books.show refuses what its target ignores. A
report asked for without dates covers this fiscal year so far, everywhere
(one rule for tools, descriptions and the viewer; the IIF and CPA package
keep their own dates). policy.require_tax_category warns or refuses; a
Form 4797 gain is shown apart on every return.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01T3gXMVt7XJPi6ens1or9Dj

README.md

@@ -100,7 +100,7 @@ just          # lists the recipes: check, build, prove, demo, ...
 ```
 crates/ledger-core/    # engine: money, accounts, journal, ledger, reports
 crates/ledger-store/   # journal + SQLite projection + blob store
-crates/ledger-mcp/     # MCP server: 108 tools, resources, handbook, receipt policy
+crates/ledger-mcp/     # MCP server: 109 tools, resources, handbook, receipt policy
 crates/ledger-import/  # QBO CSV import, IIF/XLSX export
 crates/ledger-config/  # wanderbooks.toml company settings
 crates/ledger-cli/     # `wanderbooks`: the one executable (every mode)