docs(fred): README, design doc, marketplace entry

[walkerhughes]

Closes #35, and completes the stack from #30. On top of #39. No new code in src/.

This is the PR that makes the plugin installable, which is why it comes last rather than first: the stack should not advertise a server whose tools do not exist yet.

What's here

• plugins/fred/README.md — install, credentials, the five tools, development.
• plugins/fred/docs/design.md — the reasoning, following plugins/tastytrade/docs/design.md.
• .claude-plugin/marketplace.json — the fred entry.
• Root README.md — a row in the servers table, and fred in the layout tree.

What the curation buys, measured

The design doc's central table is measured against the live API, not estimated. Response sizes in characters:

Question	Endpoint-wrapper baseline	This server
Fed funds rate now, and its 20-year range	1 call, 694,388 chars	1 call, 4,194 chars
CPI year-over-year vs unemployment since 2015	2 calls, 27,758 chars, unaligned	1 call, 6,777 chars, one index
Find the unemployment rate series	1 call, 8,201 chars for 5 results	1 call, 1,835 chars
What did Q3 2025 GDP first print at?	1 call that 400s, then guesswork	1 call, 9 vintages collapsed to 1 revision

The first row is the argument for the whole design: 0.6% of the payload, and a better answer, because the exact 20-year minimum of 0.04 and maximum of 5.41 are in the summary rather than somewhere in 5,000 rows a model has to scan.

Two guards this repo has needed before

• ${CLAUDE_PLUGIN_ROOT} must be bare. The :-default form is handled by env-var expansion, which does not know that variable, so the default always wins and the server launches against the user's own project directory. tastytrade carries the same test with a note that a sibling plugin shipped broken three times before it existed.
• The marketplace entry's source must resolve to this directory. A listed plugin pointing at the wrong path fails at install with nothing useful to read.

Version ladder

The plugin version gate compares each PR against its own base, so every PR in a stack that touches shipped files needs its own bump. Applied bottom-up:

PR	Version
#36 scaffold	0.1.0
#37 discovery	0.2.0
#38 observations	0.3.0
#39 revisions + calendar	0.4.0
this one	0.5.0

Nothing is installable until this PR lands, so no intermediate version ever reaches a user.

Verification

284 tests, 98% coverage, make check clean.

End to end through the real launcher, minimal PATH, real API key:

server: {'name': 'fred', 'version': '0.5.0'}
tools:  ['get_observations', 'get_release_calendar', 'get_revisions', 'get_series', 'search_series']
live:   get_observations(UNRATE,CPIAUCSL, units=yoy, start=3y)
        -> 35 dates, both series on one index, UNRATE yoy latest 2.43902

The version reported over the wire comes from plugin.json rather than a constant, so the two cannot drift.