docs(local): make `glab orbit local` the primary install path

What

Three Orbit Local pages still told users to "build from source until packaged binaries ship". They ship: glab orbit local --install --yes installs the managed orbit binary (v0.58.0) in glab 1.94+. This flips the install guidance and corrects the related glab orbit setup framing.

Changes

• getting-started.md - drop "planned" from the access-method table; rewrite the 60-second quickstart note to describe the binary download on first use.
• access/cli.md - new Install section leads with glab orbit local --install --yes and the one-line installer; build-from-source demoted to a "Build from source" contributing note.
• access/glab.md - reframe glab orbit local and glab orbit setup as shipping today; add an Install section; replace the fictional setup flags (--agent/--skill-only/--mcp-only/--dry-run) with the real ones. Keep glab orbit local mcp serve and glab orbit local status marked planned - neither exists in the v0.58.0 binary.
• _index.md - drop "(planned)" from the glab access row.

Verification

Verified against the shipping CLI (orbit v0.58.0, glab 1.101.0):

Command	Result
glab orbit local --install --yes	works
glab orbit local index / schema / sql / query	work
glab orbit setup	ships (top-level command: verify access, install skill, install local binary)
glab orbit local status	unrecognized subcommand - left planned
glab orbit local mcp serve	unrecognized subcommand - left planned

Existing sql/query examples kept as-is (they work). The originating issue suggested glab orbit local query -, but v0.58.0 takes the query JSON as a positional argument, not stdin via -, so that was not introduced.

mise run lint:docs passes on all four changed files (the 3 pre-existing vale errors are in unrelated files).

Out of scope (follow-up)

access/mcp.md has separate inaccuracies not touched here: it calls glab orbit setup "planned" (it ships) and tells users to run orbit mcp serve / glab orbit local mcp serve to serve the local graph - but neither subcommand exists in the binary. Worth a separate issue.

Closes #668 (closed)

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