cli: `waybackify manifest --source <url>` populates `sources`

Problem

After #8, a manifest can carry sources. There is no command that writes the key. waybackify manifest resolves the links inside a source file and writes entries. A user who knows where a document first ran must edit wayback.json by hand, and must find the Memento by hand.

Proposed solution

Add a --source <url> flag to waybackify manifest.

• The flag names one Original Resource of the document. It repeats for a merged document.
• The command resolves each URL to a Memento with the same resolver, --near hint, and --offline rule that entries use.
• The command writes each result under sources in the output manifest. entries are not affected.
• When the manifest already has sources, the command merges. A URL already present keeps its Memento. A new URL is added. Nothing is removed.
• When a URL resolves to no Memento, the command reports it and exits non-zero, the same as an unresolved link. It does not write a partial sources map. (If #8 decides that an unarchived source goes in exclude, the command writes that instead.)

Example:

waybackify manifest post.md -u universe.json -o wayback.json \
  --source http://blog.nodejitsu.com/a-simple-webservice-in-nodejs

Acceptance criteria

• With --source <url>, the output manifest has one sources entry with a parseable wayback and timestamp.
• With two --source flags, the map has two entries in sorted order.
• Rerun on the same file with the same flag: the manifest does not change.
• Without the flag, the command output is identical to today.
• --help documents the flag.

Verification

• Tests pass: new cases in src/waybackify-cli/test/manifest-command.test.js, and the help fixture regenerated.
• pnpm -r test passes offline.

Dependencies

Blocked by #8.

Files likely touched

• src/waybackify-cli/src/commands/manifest.js
• src/waybackify-cli/src/cli.js
• src/waybackify-cli/test/manifest-command.test.js
• src/waybackify-cli/README.md

Estimated scope

Small (3-4 files)

Alternatives considered

• A new waybackify sources add command. Rejected. The resolution is the same work that manifest already does, and one command keeps one write path to wayback.json.
• Read the source URL from the markdown frontmatter. Rejected. The library has no frontmatter convention, and the flag keeps the input explicit.

Non-goals

• Fetching the capture. That is #9.
• A bulk mode over a content tree. A shell loop over the manifests covers the first consumer.

Context

• Command: src/waybackify-cli/src/commands/manifest.js.
• First consumer: the charlie.dev backfill of every historical post.

After dependency #8 is resolved, start with src/waybackify-cli/src/commands/manifest.js and src/waybackify-cli/src/cli.js, then read the existing manifest tests. Add coverage in src/waybackify-cli/test/manifest-command.test.js and regenerate the help fixture. Done means source URLs resolve and merge without changing entries, failures avoid partial writes, help documents the flag, and pnpm -r test passes offline.