v7.0.0

Added

• Feat!: rename fixer agents by concurrency model (parallel / serial)

BREAKING: both Opus-class fixer agents are renamed to spell out the
fundamental design distinction — concurrency — directly in the name:

llm-externalizer-fixer-agent -> llm-externalizer-parallel-fixer-agent
llm-externalizer-bug-fixer-agent -> llm-externalizer-serial-fixer-agent

The pair "fixer" vs "bug-fixer" was ambiguous — both agents fix bugs,
and a reader couldn't tell from the name which was which. The real
design axis is how they execute:

• parallel-fixer-agent: stateless, writes a .fixer. summary per report,
  dispatched up to 15 in parallel against a folder-wide scan
• serial-fixer-agent: stateful on disk (mutates the aggregated bug
  list with " — FIXED" markers), dispatched one at a time in a loop
  over one bug list

Reviewer-agent name is unchanged (it's read-only, not a fixer).

Touched everywhere: agent files (git mv + frontmatter name: field +
internal BACKUP path prefixes + [FAILED] messages + example dialog),
command Task dispatches and descriptions, README features + commands
table + plugin structure, scripts (fix_found_bugs_helper.py help text,
validate_fixer_summary.py docstring). CHANGELOG entries are historical
commit records and were left untouched.