docs(spec): command log tenant_id exception, bump to v0.5

[OriginDevIT]

Closes the last of the four spec gaps opened by the command layer session, and bumps the header.

• §11.2 now states that the command log row is written before the command transaction and concluded after it settles, each independently committed. A log inside the transaction it is logging vanishes on rollback, which is the case it exists for.
• command_log's nullable tenant_id is stated as a bounded exception, with three conditions: it records an attempt rather than a state change, null rows are unreadable by osds_app, and they are written already concluded so the app never needs to update one.
• A new exception is a maintainer decision recorded in docs/decisions.md, not something an implementer claims.
• CLAUDE.md invariant 3 names command_log and points at the rule, so the migration no longer reads as a violation.
• The access log in §11.2 is left alone; it inherits the rule if and when it is built.
• Header bumped to v0.5 with a changes block covering §4.3, §7.1, §9.5 and §11.2.

Closes #49