feat(ultraplan-local): M0 — profile foundation, no behaviour change

Introduces a profile-loader infrastructure for runtime-instantiable
ultraplan variants (depth × domain × goal axes). M0 ships only the
`default` profile, which mirrors the current hardcoded Phase 5/9 agent
set — so existing flows are unaffected.

What lands:
- profiles/default.yaml — schema v1, lists current 8 exploration agents
  + 2 review agents, captures today's adversarial regime
- scripts/profile-loader.mjs — null-deps Node loader with limited-subset
  YAML parser, listProfiles(), loadProfile(), validateProfile() that
  cross-checks every referenced agent exists in agents/
- scripts/profile-loader.test.mjs — 26 node:test cases (parser, validation,
  loader, integration with built-in default.yaml)
- commands/ultraplan-local.md — Phase 1 gains a "Resolve the profile"
  step (--profile flag → brief.recommended_profile → default fallback)
  and prints profile + source in the mode report. Phase 5/9 unchanged.
- README.md, CLAUDE.md, marketplace README — documentation of the M0
  foundation, the universal-brief design principle, and the M1/M2/M3
  milestones to come.

M1 (next) wires profile recommendation into ultrabrief Phase 4. M2
ships the additional built-in profiles (quick, bugfix, feature, refactor,
security-deep, research-heavy) and replaces the hardcoded Phase 5 agent
table with profile-driven selection. M3 adds user-extensible profiles.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

plugins/ultraplan-local/commands/ultraplan-local.md

@@ -91,7 +91,12 @@ Parse `$ARGUMENTS` for mode flags. Order of precedence:
 7. **`--quick`** — set **mode = quick**. Skip agent swarm; use lightweight
    Glob/Grep scan and go directly to planning + adversarial review.
 
-8. If neither `--brief` nor `--project` is present after flag parsing,
+8. **`--profile <name>`** — explicit profile selection. Overrides any
+   `recommended_profile` from the brief frontmatter. Set
+   **profile_override = {name}**. Validation of the profile happens in the
+   profile-resolution step below; an unknown name aborts with a clear error.
+
+9. If neither `--brief` nor `--project` is present after flag parsing,
    output usage and stop:
 
 ```
@@ -100,6 +105,7 @@ Usage: /ultraplan-local --brief <path-to-brief.md>
        /ultraplan-local --brief <path> --research <research-brief.md>
        /ultraplan-local --project <dir> --fg
        /ultraplan-local --project <dir> --quick
+       /ultraplan-local --project <dir> --profile <name>
        /ultraplan-local --export <pr|issue|markdown|headless> <plan-path>
        /ultraplan-local --decompose <plan-path>
 
@@ -111,6 +117,8 @@ Modes:
   --research    Add up to 3 extra research briefs as planning context
   --fg          No-op alias (foreground is the only mode as of v2.4.0)
   --quick       Skip exploration agent swarm; plan directly
+  --profile     Explicit profile (overrides brief's recommended_profile).
+                List available with: node profile-loader.mjs list
   --export      Generate shareable output from an existing plan (no new planning)
   --decompose   Split an existing plan into self-contained headless sessions
 
@@ -119,6 +127,7 @@ Examples:
   /ultraplan-local --brief .claude/projects/2026-04-18-jwt-auth/brief.md
   /ultraplan-local --project .claude/projects/2026-04-18-jwt-auth --research extra.md
   /ultraplan-local --project .claude/projects/2026-04-18-jwt-auth --fg
+  /ultraplan-local --project .claude/projects/2026-04-18-jwt-auth --profile default
   /ultraplan-local --export pr .claude/plans/ultraplan-2026-04-06-rate-limiting.md
   /ultraplan-local --decompose .claude/plans/ultraplan-2026-04-06-rate-limiting.md
 
@@ -144,12 +153,52 @@ If `research_status == pending` and `research_topics > 0`:
 - If cancel: print the research invocations from the brief's "How to continue"
   section and stop.
 
+### Resolve the profile
+
+After reading the brief, determine which profile this plan run should use.
+Profiles describe which exploration/review agents to spawn, which
+adversarial regime to apply, and which catalog filter the architect should
+use. The full schema lives in `${CLAUDE_PLUGIN_ROOT}/profiles/`; the loader
+is `${CLAUDE_PLUGIN_ROOT}/scripts/profile-loader.mjs`.
+
+Resolution order (highest precedence first):
+
+1. **`--profile <name>` flag** — use it. If the profile cannot be loaded,
+   abort with a clear error listing available profiles:
+   ```
+   Error: profile '{name}' not found.
+   Available: {comma-separated list from `profile-loader.mjs list`}.
+   Use --profile=default to fall back.
+   ```
+2. **Brief frontmatter `recommended_profile`** — if present, use it.
+3. **Otherwise** — use `default`.
+
+If the resolved profile name is `default`, Phase 5 keeps its existing
+size/conditional logic (convention-scanner only for medium+, research-scout
+only for unfamiliar tech). For non-default profiles (M2 onward), Phase 5
+will run exactly the agents listed in `agents.exploration`. **In M0 only
+`default` exists**, so resolution effectively always lands on `default` and
+no behaviour changes.
+
+Load the resolved profile:
+
+```
+node ${CLAUDE_PLUGIN_ROOT}/scripts/profile-loader.mjs load {profile_name}
+```
+
+Capture the JSON output into **profile** state. If the loader exits non-zero,
+abort with the loader's error text.
+
+Set **profile_source** to one of `flag | brief | default-fallback` for the
+mode report below.
+
 Report the detected mode:
 ```
 Mode:    {foreground | quick | export | decompose}
 Brief:   {brief_path}
 Project: {project_dir or "-"}
 Research: {N local briefs, M extra via --research}
+Profile: {profile.name} (source: {profile_source})
 ```
 
 ## Phase 1.5 — Export (runs only when mode = export)